You've seen them - from the single knob epiphone Valve Junior to the full eq on a BlackHeart Little Giant to the fully-featured Mesa/Boogie Trans Atlantic. For years people have been exploding eardrums to drive their output tubes to saturation. Someone along the way realized that making a lower wattage tube amp means that you can crank those tubes up without going deaf before you can hear the tone you're after.
I personally like the idea of low wattage amp heads - they're small, they let you use whatever speakers you want with them, they won't break your back [or the bank!]... and in a pinch they serve as an excellent conversation piece. For the money you spend on that 100 watt amp, you can get 4 or 5 little amp heads, each with their own unique sound! Sure they won't blast a hole in the wall but they're plenty loud to jam with.
They are still a ways to go with making these little monsters live-friendly. Not many of them support channel switching via footswitch, or even effects loops. With the advent of affordable home recording, players are now offered a multitude of low-wattage amps at a small studio price.
For a test, recently I jammed using only small amp heads - each running through a Marshall 4x12. All 3 of them were loud enough to play in my rock group - an Epiphone Valve Junior, Orange Tiny Terror, and even my solid state Marshall MG15MSII head.