Yeah you need one or the pump/wire will get beat up big time.
No they don't keep the pump from coming loose from the pipe, they keep the pump from slamming around in the well when it goes on & off. They also help keep the wire from getting chaffed.
We always put one over the wire splice just above the pump and open it enough to drag the well bore. Depending on the type of pipe, depth and horsepower we sometimes use as many as 4.