Throwing footballs is something my daughter's coach incorporates in her sessions with him. Not only does it simulate the service motion, it helps her to feel the load in her shoulder blades just below her neck. It also helps with the wrist snap that you need in a good serve. A good spiral is comes from a good wrist snap. Throwing footballs also helps in tracking balls from a static position then exploding to reach/catch the ball by either backpeddling, cross over stepping, with sudden change in direction...all of which is part of tennis.
"Can we all just get along" Rodney King