On a daily basis, working the same muscles to the point of failure is a quick way to get hurt. But the whole point of working out is to shock the muscles into building themselves up.

This may be a little oversimplified, but you tear a muscle down by working it. It realizes it's not strong enough to do what you're asking it to do, so it rebuilds. The fastest way to convince your muscles that they're not strong enough is to work them to the point of failure. But that technique should be used sparingly, and if you don't get enough recovery time in between workouts, your actually doing more harm than good.