Have you ever tried to hit a moving target? It's a lot harder than shooting a stationary paper target at a shooting range.
Similarly, criminals (e.g., drug dealers) don't stand still. Moreover, criminals can sometimes work in pairs if they want to rob or assault someone.
Hence, you better hope your ten bullets in your typical hand gun hits the attacker while the attacker is moving, which is not an easy thing to do when under stress.
Likewise some defenders may need more than ten bullets to hit their attacker(s), especially if there are two or more. Accordingly, large capacity magazines can help hit moving targets (e.g. career criminals) while under stress.
