Here’s another Q&A from "The Baking Answer Book" by Lauren Chattman.
QUESTION: I’ve had some bad exerpiences trying to remove hot cookies from a baking sheet. Why can’t I just let my cookies cool completely on the sheet?
ANSER: There is a good reason why most recipes instruct you to remove the cookies from the baking sheet immediately or after just a few minutes outof the oven and transfer them to a wire rack. Cooling the cookies on the rack allows air to circulate around them. If cookies cool on the hot baking sheet, moisture from the cookies will condense on the pan and seep back into the cookies, making them soggy.