I'm going to go with "Needs of the (insert branch of service)..." I can't speak for SF, but I have a funny feeling that if they spent all that time and money to train you for your occupational specialty and a
geographic region, it stands to reason an E5 will spend quite a bit of time at Group (insert Group whose AOR you received your training on)...
The Corps isn't there quite yet, it's a 5 and fly