Because driving stakes with a rock ruins the stakes and eventually your hand.

A proper mallet gets the peg in straight and fully, which is most of what holding power comes down to. A peg driven at an angle with a rock is half as effective and twice as hard to pull out afterwards.

Most of these have a hook or claw on the handle end for extraction — which is the half of the job nobody thinks about until they are prying at a buried stake with a tent pole.

Lives in the awning bag, not the toolbox. You want it where the stakes are.