I never knew anyone who was into diving and took ballet.
My guess would be for the power of centering and to strengthen the legs.
Boxers take ballet for the footwork, but also for the centering. Football players take ballet for the footwork, but also for centering. Track people take ballet for the ability to jump (like for hurdles), but also benefit from the centering.
My belief about about how ballet benefits athletes is in the centering. You get better body control when you can center your weight.
The one group that I have never heard of taking ballet happens to be surfers. But if you watch surfers, the come really close to the body control a ballet dancer needs. They can manage their centers so well, it's amazing.
balletboy[/i]