Most peanut butter cookies have a criss-cross pattern on top that's made with a fork. This pattern is made to flatten the cookies so they cook evenly. The fork marks also add crispness, as the thin edges created by the fork crisp up when baked, much like the edges of the cookies. And finally, the indentations made by the fork also do a good job capturing the sugar that gets sprinkled on top, adding even more crispness.