I would like to point out that 'guitar amps' and 'hifi amps' are two different beasts. Hifi, for reproducing 'music' or sound accurately are designed for the lowest distortion possible, with flat response. Guitar amps (be they valve, or solid state) are designed to purposefully colour the sound - significantly. The area of guitar amps appears to be the bulk of the 'valves vs solidstate' debates all over the internet.
We're not talking 'distortion' as an added effect for metal/rock, or overdrive although that
is designed into the amp and speakers. Even when a guitar amp is 'clean', it exhibits much greater signal distortion over hifi amps. You can try this yourself - plug your guitar into a buffer (for impedance) then A/B it into a guitar amp and speaker, and hifi amp with its speaker. The hifi amp will sounds teribly stale, cold, and generally unpleasant when compared to the guitar amp - thats because that
is what the guitar signal is
A guitar amp/speaker will amplify harmonics, even when well below the 'overdrive' threshold - sounding perfectly clean. Also note: guitar amp speakers are designed to enhance this - limited frequency respose in the mid-range.
Whilst most of us electric guitarists love distortion/overdrive (there's a massive culture/debate/discussion/religion about it on the web) many forget about a clean tone, and somehow assume its just the guitar signal 'but louder'. It really isn't. Otherwise all guitar amps would sound the same when clean, and there wouldn't be countless digital effect processors all with hundreds of 'clean amp modelling' (otherwise, the clean tone wouldn't require any processing!).
So, not trying to get off track here, after all this is all 'valves vs solid state' - but for electric guitars or basses, they serve to change the sound as well as amplify, so they have the opposite goals of a true 'amplifier'. tvtech, its easy to get confused when words like 'distortion' is used. In guitar amps, it generally means crunch/grind where the amp is designed to have high distortion, with a low clipping threshold, and distort 'musically'. In true electronic amplifiers, that is something which is to be avoided, at all costs. I have probably over-simplified it, after all many have gone into great detail, but essentially, thats what I understand, and agree with.
I haven't even touched on 'why' what with modern processing guitarists still use unreliable, heavy, expensive valve amps - its just because historically, electric guitars have been used with them, because at the begining, they were the only amps available, and we've all grow up listening to guitar music made with valve amps (or music influenced by music made with valves)