On most ovens, it's just a storage drawer. The warming drawer is an upgrade feature on certain models, and actually has its own heating element separate from the main oven.

Friends just put in a new kitchen, and one of the things is a warming drawer above the oven. (Microwave above that.) So when you're using the over, it warms automatically. If you're not using the oven, well, I don't know, I didn't think to ask.