It's so a lock can be put on it to lock it on or off.

YES, the breaker can still trip if it is locked on.

Yes, it's a lock out. Usually it is done when that breaker is the maintenance disconnect for some piece of equipment but it is not "in sight of" the equipment it is controlling. I've got one in my townhouse because this county thinks the wall ovens need one (the other big loads like the water heater, furnace, A/C compressor, all have disconnects located right at them).

Thanks. I called my inspector and he told me it was not necessary. He couldn't figure out why the lock was there in the first place on the furnace.

Thanks all!

If your furnace doesn't have another disconnect and you can't see this breaker from the where you work on the furnace, then it needs to have a lockout provision.

Otherwise, it's spurious/

Yep, that was it. Turns out there is no shut off at the furnace (I never even noticed!). Seems weird that they didn't install one.

I put the furnace on the tandem circuit and I'm just going to put the lock on there too.

Or just put a switch or pull-out disconnect at the furnace. Despite what the code permits, I like to be able to see my disconnect when I'm working on things.

Does not the furnace require its own dedicated breaker?

It's on a dedicated breaker, just a tandem now instead of a solo.