Its all about image and the very clever marketing ploy by all the big boys - Marshall, Mesa etc to have you buy amps that you really cannot use until such time as the power tubes are starting to glow hot and then comes the sweet tones. In my early days I have had Marshall, Hiwatt, Soundcity stacks and although nice to be honest you played so loud that it was difficult to get a good tone acroos the whole band because of the imbalance. This was at a time when PA systems were frankly crap!!
You really don't need 100w amps unless you are gigging in stadiums, large stages - most of the "stage gear" that you find are actually dummies - and no sound is coming at all, just there to say oh how impressive!! Most of the time I use Blues Junior linked upped and off the floor on stands, other times I will use my Nomad 55 and ext speakers but again these are off the floor (see the picture below for an example of what I use).