Tube (valve) amps have a different tonal quality from solid state amps, which is preferred by many guitar players. The classic tube sound is produced when a tube is overdriven and distorts the waveform. Valve distortion is characterised by 'soft clipping' as well as other effects which are explained in Merlin Blencowe's book 'Designing Tube Preamps for Guitar and Bass'.  It is difficult to achieve these effects using solid state electronics.

The picture shows soft clipping distortion  (upper trace) ; the lower trace is the undistorted signal.  The amount of distortion used is entirely a matter of taste, but the scale below shows the degree of distortion typically used in musical genres: