1. It is aesthetically pleasing so that many artists have portrayed it quite beautifully, which is impossible with the anuses of animals, right out in the open.
2. The cheeks hide and protect the anus when at rest.
3. the cheeks part when pooping to let the poop slide past with a minimum of touching. Of course, some still touches, so we still have to wipe.
In addition, of course, our bottom is comfortable to sit on. No particular question, just musing on a wonderful thing.