Quite obvious really isn't it.....
You just use a little more stabaliser muscles in standing, but can be a little looser on form too.
You could also ask:
Whats the difference between standing dumbbell curls and seated dumbbell curls....
Performing the press sitting down allows one to focus on their shoulder, one must be careful not to arch the back and not lock elbows at the top of the movement.
is arching the upper back fine? Jim Wendler - Military Press 225x11 - YouTube