1. What if you get injured while on the mission? Having a scholarship waiting would give you time to recover once you returned.
2. If you're not a top notch recruit, you'll want to take the scholarship being offered while you can. You might find it gone when you got back.
3. BYU is in the Big12 when you get back and you want in on that party!
I'm sure there are other scenarios. Certainly each situation is different.