When a creature is prone, crawling is one of the only ways it can move.
Action: Move action. A creature must be prone to take this action.
Move: The creature moves up to half its speed.
Standing up and then moving is usually preferable to crawling, but sometimes a prone creature wants to move and doesn’t have enough actions to stand up first. In that situation, crawling and teleporting are the best options.
Published in Player's Handbook, page(s) 288, Rules Compendium, page(s) 241.