While throwing for hunting purposes would have been useful, throwing for defensive purposes likely would have been the initial impetus for throwing evolving to a fine art.

Imagine yourself alone on the plains of Africa and facing a lion or two. Your odds are not good. Even a group of people on the plains of Africa facing a few lions with no ability to throw stones looks like a losing proposition.

However a group of people in an outward facing circle facing lions and each with a pile of stones to throw with deadly accuracy, now this is something that even lions would think twice about.

The thing about defence is you don't have to move, the stones are prepared before hand. The predator has to come to you and is therefore easier to hit, preferably in the head. Sticks and stones will break their bones but names will never hurt them.