I think it's because most people don't have the luxury of having several feet behind their background to fire lights. In most cases we try to get the background as close to the wall as possible so that we can get separation between subject and background.
I also think (and Allen can correct me if I'm wrong) that it takes a bit more firepower to properly illuminate the white paper from behind whereas it doesn't take all that much to do it from the front.
Yes you will need some power. I think in the family image I had two White Lightning 1200s at about half power into the background and that would have given a reflective reading from the front of the seamless of about f/22 or so and the light on the subjects (incident flash reading) would be about f/8 giving the skin tones if meter reflective about f/11 so the white is PURE white two stops up from the skin and 3 stops up from the mid gray exposure. The would put the skin at zone IV and the white at a zone VIII.