I have a strange question...why do our bottoms have a big crack down the middle? Why isn't it a smooth surface with a little bum hole, and what reason do we have bum cheeks that stick out and are cushiony????
I'm guessing that your legs wouldn't be able to move as they currently do if your bum was flat. They are generally "padded" to allow primates to sit upright comfortably without resting weight on their feet.
It has, primarily, to do with the anchoring of the gluteal muscles and other structures that allow humans to stand upright and walk. The gluteus maximus is the largest muscles in the human body. By the way, the beginning of the cleft at the top of the separtion ("plumber's crack") was once called nock... same as the nock of an arrow where the bowstring sets...
never heard nock before but it's doubtless the same word as notch (same way 'bake' and 'batch' are related). I imagine it was invented as somewhere for young women to hide their thongs.