Covers sensible cases as well though;
When a keeper is kicking to release the ball, or throwing the ball, he is controlling it with one hand.
Sometimes it is better for a keeper to jump to reach for a ball with one hand than it is with two, for more distance.
I also expect, it was put in because we had referees trying to rules lawyer it and say "well, it wasn't both hands so it's no foul" or "Oh well he only had a few fingers on it", as usual, so this completely shuts down that type of referee and makes it clear what is expected of them.