Have used one in the bottom of my GE oven for years. Lays on the floor of the oven, not the rack. It is one of those specifically for this purpose, don't remember the brand name, think I got it at Bed Bath and Beyond. It is great. Any overflow or spill is much easier to clean up, just lift it out and wash it off.
Makes oven cleaning much easier too, only have to worry about the sides of the oven. Haven't figured out how they get so spotty but they do...

I've kept one on the oven floor, under the heating element, in my past 3 ovens and love them. You can cut the oven liner to fit the size of your oven.

-Grainlady

Agree with the others. Won't do without an oven liner, they are great. They go under the heating element and catch any spill overs from pies, casseroles, etc. NancyLouise

The aluminum ones lay on the oven floor, which is what I've used for years. That's if you have an open element. The ovens where the element is hidden, these are not to lay on the floor, as the heat can weld it to the oven floor.

They have the siliconish ones now, which say to lay on the oven rack.

Even though I use a cookie sheet under things that have a large potential to spill-over.....scalloped potatoes, fruit pies, something else can spill over and the oven liner is a saving grace.

If they would only invent something for the wall.

I have 2 ovens, one self cleaning, the other not ( what was I thinking!!). I have a silicone liner in the non self cleaning one.
A foil or aluminum liner changes the way your oven cooks...I wouldn't use one.
Linda C

Be very careful. What does the operating manual say.

Many ovens now have a special coating and the aluminum will bond to it and can't be removed.

Every now and then you'll see a plea from someone on the Kitchens or Appliance forum about how they ruined their brand new oven the first time they used it because they put a liner on the bottom, despite clear instructions not too use a liner or foil on the bottom of the oven.

I bought one from the dollar store (an aluminum one) and it gave off such a bad scent when I preheated the oven I took it out and haven't used it since. I keep it for other things. I like the idea of having one though so I will look into a reputable source for one I guess.

I've used them for YEARS with NO difference in baking/cooking/roasting. I buy them, very inexpensively, from Walmart and love them. One less surface to have to clean. I think they come two to a pak.