How come bowls, cups, plates, etc. have ridges on the bottom?

In art class we’re working with clay, and I’m making a teapot. Right now, the bottom of my pot is perfectly flat, but, looking through my kitchen, it seems like every piece of ceramic foodware has a thin ridge going along the bottom, with the rest of the bottom indented somewhat.

Example One
Example Two
Example Three
Example Four
Example Five

Mine looks sort of like this at the moment.

Is there a pragmatic reason for the ridge, or is it just aesthetic? Should I put a ridge on the bottom of my teapot?