Some springform pans have a perfectly flat bottom piece, but other springform pans have a bottom piece with bumps all over it, such as this one:
Do the bumps serve any purpose? I have never used a springform pan before, but I would imagine the bumps might make it more difficult to completely slice all the way through a cheesecake, resulting in a lot of crumbs breaking off the crust as you try to remove a slice of cheesecake from the bottom piece. I would imagine that fewer crumbs would break off the crust if the bottom piece was flat.