I doubt that the scenario you imagine is the correct one-- but it is possible. More likely is that this was installed in the early days of computers and an isolated/dedicated ground was desired in addition to the circuit ground. Since the branch circuits in that house had no grounds at all, that makes it even more likely.
The belief was that circuit grounds could have "noise" on them from the other loads on the circuit.
While this is in a house, many industrial/commercial premises are wired per specifications of an engineer. Often they specify an equipment grounding conductor even if the conduit system is recognized as providing the equipment ground. The practice is definitely not a code violation.
In your case, think of it as a belly button - you have it and might as well keep it.

but the panel has over 16 conduits running from it to everywhere and no where else has a green wire except this one.

Plus, this green wire is now connected to a ground screw in this junction box, and there is no fixture tied to this box. It's just a bunch of wires spliced and runs to somewhere else. Does the green wire really serve a purpose in this case? I am thinking it may be a safer thing to have each existing circuit a dedicated neutral wire, rather than a shared neutral with a seemingly redundant green wire.

Where did you get the idea that green has anything to do with neutral?

I didn't.

I just find it odd that amongst all the conduits that runs from the panel that only one has a green ground wire going to a junction box with no fixture.

and in that box I have two circuits sharing a neutral...and an extra green wire.

Just the two together makes me wonder if this green wire might have been once used as a neutral wire, and the two circuits used to have their individual neutral wires.

"Does the green wire really serve a purpose in this case?"

No. Although I doubt you have rigid. Rigid has threaded ends on the pipe. So, at a box there is no coupling. Just a locknut inside and a locknut outside.

EMT has a coupling. Usually with a set screw.

"I am thinking it may be a safer thing to have each existing circuit a dedicated neutral wire, rather than a shared neutral with a seemingly redundant green wire."

No, that would be less safe and against code. Green wires may only be grounds.

Two hots with a shared neutral is an elegant system called a MultiWire Branch Circuit. The neutral carries the difference between the two hots. For instance with a 10 amp hot and an 11 amp hot the neutral would carry one amp. It runs cooler, with less stress on the insulation, than the same thing with two neutrals.

The two hots must be on opposite 120V legs. What many people call "phases." This is most safely done with a handle tie or by using a double pole breaker.

Does this by any chance feed a spa, pool, or hot tub?

No it does not feed anything. The green wire runs along a conduit to a junction box where some splicing was done, the green wire is attached to that metal junction box with a green screw. No fixture, nothing. But in this junction box, a neutral wire is split into two, run into another conduit with two hots from two circuits. There are dozens of conduit runs and only this one has a green wire in it. This 75' long green wire one end is on the panel the other end terminates in a junction box, may be it's a redundant ground.

"Two hots with a shared neutral is an elegant system called a MultiWire Branch Circuit. The neutral carries the difference between the two hots. For instance with a 10 amp hot and an 11 amp hot the neutral would carry one amp. It runs cooler, with less stress on the insulation, than the same thing with two neutrals.

The two hots must be on opposite 120V legs. What many people call "phases." This is most safely done with a handle tie or by using a double pole breaker."

The thing is I only have single phase electric, so I don't see how MWBC is even possible in my case. So if the two circuits have a share neutral, the neutral wire is taking the combined loads right? The breakers are not adjacent to each other either. Granted this was back in the 70s when it was wired under who knows what code...

That's why I wondered if someone had originally used the wrong green color for neutral wire because they ran out, and later on someone else opened up that box and uncoupled the green and created the single phase shared neutral circuit.

So basically right now I have a shared neutral in a single phase. I have an seemingly unused wire but of a different color. I was going to pull a new white conductor for this...then I thought, wait a minute, what is this green wire doing here?

Holy cow!!! What is the problem with having an equipment grounding conductor in the conduit? Nothing is wrong with the install. You act like the fact that a green insulated wire is tied to a green ground screw is a problem. It is not a problem. What size breaker are the wires landed on? If it is a 50a then it may have fed a hot tub at one time. In such a case, an insulated equipment ground conductor is REQUIRED. You have 2 hot wires sharing a neutral. As long as they are on opposite hot legs (phases) you are fine. Yes, you have single phase power, not 3 phase. That does not mean you only have one hot leg in your panel. You have 2 hot legs. You don't understand enough about electricity to even know there is not a problem with the install at all. It is most likely perfectly safe and legal.

Do more research about MWBC. The present lack of understanding is part of the problem.

"The thing is I only have single phase electric, so I don't see how MWBC is even possible in my case."
Single phase has two legs. They are perfectly matched to make a MWBC.
" So if the two circuits have a share neutral, the neutral wire is taking the combined loads right?"
On the same leg, loads would be combined. On opposite legs they cancel.
"The breakers are not adjacent to each other either."
That's exciting. But grandfathered in. So ok, as long as nobody screwed up the legs.

I hesitate to stick my nose into this one but here's a good example of why I don't like MWBCs in a residential setting. As Glen pointed out the breakers need to be on opposite legs, otherwise the current carried by the grounded conductor (neutral) is the sum of the two loads and potentially dangerous.

The OP indicated that the two breakers are not adjacent and that's something that should be corrected. First to ensure they are on opposite legs and second to tie the handles.

As for the green wire...who the heck knows. Isolated ground? Belt and suspenders? I've seen EMT connection that have come apart (mostly due to things like not clamping it correctly). In one house they were having foundation problems. That section of pipe had obviously installed after the problems had started because when the fixed the foundation, they pulled to sections of pipe apart. Somebody who wasn't familiar with metal conduit? If you weren't very experienced and learned "Romex" you might think that a ground wire is required.

On the other hand, every green wire I've seen in residential conduit as be hot. So the OP is ahead on that score. ;-)

"The OP indicated that the two breakers are not adjacent and that's something that should be corrected. First to ensure they are on opposite legs and second to tie the handles."

It doesn't need to be corrected because it is not a problem. It was code compliant to share a neutral between two phases. They didn't have to be adjacent to each other until the "handle tie" rule was adopted. It was most likely code compliant when installed. Just because the home owner is sticking his nose into something he doesn't understand doesn't make the install wrong.

"It was most likely code compliant when installed."
Perhaps not. For a very long time the code has required that the motion of the hand that disconnects any of the conductors of the circuit be able to disconnect all. On a MWBC,single pole breakers adjacent can do this, separated, they cannot.
The old XO series SqD/Cutler Hammer panels were arranged so that only a double pole breaker could meet that requirement.

Perhaps in some local code, but not in any version of the NEC. Prior to the 2008 code, unless the MWBC used both sides to feed a device on the same yoke (such as a split duplex receptacle), the only requirement was that the circuits originated at the same panelboard. The code allowed you to otherwise consider them separate circuits.

In the 2008 code, the simultaneous means of opening all non-grounded conductors is *ALWAYS* required. This means a two-pole breaker or a mechanical interconnection (handle tie). No "one sweep of the hand" nonsense would cut it.

This post was edited by ronnatalie on Wed, Jul 3, 13 at 11:00

"Perhaps not. For a very long time the code has required that the motion of the hand that disconnects any of the conductors of the circuit be able to disconnect all."

This is referring to single phase and 3 phase branch circuits or feeders for the same equipment. Not for MWBC.