Arkham Horror Third Edition: Deluxe Gamemat - Rules of Play says:

This 3' x 3' slip-resistant gamemat immerses you in the twisted world of eldritch abominations, providing a full space to lay out the sprawling labyrinth that is the city of Arkham. Not only that, but the mat is the perfect way to organize your game, with room for the full board in the center and the various components along the edges.