Using a power soak/attentuator will shorten the life of your tubes, because you're driving them harder for any given volume. People say amps sound better when you crank them, so this is obviously what people are trying to achieve, without making their ears bleed.
They're becoming more and more redundant as amp technology increases, a lot of amps these days have variable power outputs (sometimes 1 or 2, sometimes infinite) and there's also a school of thought that all you need is a master volume control that works properly.