What is the criss-cross pattern on top of peanut butter cookies used for?
Peanut butter cookies don't spread as they cook, so you have to flatten them before hand. This ensures that the middle will cook through before the outside burns.

As for the pattern created, it actually creates slightly more surface area, so you'll get more browning at the extra edges that you create. Think of it like a meringue, or the top of a shepherd's pie -- if it's too smooth, you won't get the little bits of browned crispy bits that you'd get if you rough up the surface.