I am essentially completely incapable of throwing the ball straight. I have to wear a wrist brace because I have a weak wrist that breaks back by default normally, but even without it, I STILL can't help but put side rotation on the ball. If I don't, I can't aim very well. Throwing a spare ball lets me keep the ball on-path no matter what the oil pattern is like (great for PBA Experience league), but also lets me throw the ball how I feel comfortable doing it, so I can maintain accuracy.