Why do glasses, cups and plates have a ridge around the base?

If you imagine a plate with no ridge and there is some small piece of grit, food or dirt on the table the plate would rock and make it difficult to eat off it. However if you have a ridge it is very unlikely the grit is under the ridge itself and even if it is when you move the plate slightly the plate would fall off the grit becoming stable again.

It is also probably more difficult to manufacture a perfectly flat piece of pottery than a ridge that is the same height all the way around.