What is the purpose of a straight comb stock having a cheek piece? Just the cool appearance?

Mostly looks. Certain face and body types it will provide a better cheek weld. It also puts your cheek in a straight line orientation with the sights on stocks that have cast and toe in or out. Ones like a Whelen cheek piece with a concave sort of saddle configuration do seem to help with consistent cheek placement. A cheek piece and a wider comb can help prevent getting a bruised cheek with a heavy recoiling rifle.