I think the theater is basically the boot camp for the actor. If you can survive the rigors of an eight-show-a-week schedule and be at your best all the time, you can handle virtually everything because no other craft requires you to get it right every single time.
In theater, the show must go on, so you train yourself to be able to nail it every single time because that's what the audience deserves, and that's the magic of live theater.