It's the basis for topspin. But even if you hit flat, it helps bring the ball down. I don't know why (inertia?) but it does.
On your backhand, it's all arm. Do this: as you take your racket back, put all weight on your back foot. Step in/forward with your right foot, with your weight shifting from your back foot to your right foot. As you're stepping in, you start your swing in conjunction. As the point of ball contact, you should - by default - have enough forward momentum to crush the ball.
When their tennis really matters, Babolat Reps use a Wilson.