Tube amplifiers are still where it's at for guitars. Still not beaten despite the attempts of digital signal processing. The reason is that the same thing that makes it a bad general amplifier makes it an excellent guitar amp.
All of these things can actually result in a cool guitar sound in the right proportions. Hard rock in particular has strongly distorted sounds created by tube amps (for the most part). But even sounds which we think of as cleaner have a significant amount of tube distortion (e.g. the intro to Sweet Home Alabama, or the main riff of Day Tripper). If you put an electric guitar into a regular amplifier, in general, it sounds flat and terrible.