I have an amph that runs 0.4 watts, I have 18 watt amps, 40 and 60 watt amps, and I also have a 100 watt Marshall. There is something very strikingly different between the 100 watt head and the other amphs.

Besides the volume difference, there is a huge difference in bottom end between a 100 watt amp and a, say 18 watt amp. It doesn't matter so much for cleaner sounds, but for more of a rock tone, it makes a big difference. That is one reason you seldom see big acts (with more of a rock tone) with 18 or 25 watt amps. It's just a very distinct difference in thump and low end punch from a 100 watt amp than a smaller amp. Even between a 50 and a 100 watt Marshall, this is clearly distinguishable.

As for me, I do play cleaner stuff most of the time, so I don't really NEED to use my 100 watt amp all the time. I have it because I got it for a steal used, and I do immensely enjoy playing it once in a while, for that young rocker in me!