In recent years Mini and Micro amps have become popular with musicians both for their novelty value and for the fact that they are so small and compact with a very low wattage rating.
Compact & Portable Headphone Amplifiers
Some are not much larger than a packet of cigarettes and the concept is that they can be easily transported and because they operate on batteries can be used on an "anytime, anywhere" basis. The typical volume for these mini amp units is only one or two watts but they do allow musicians to plug-in and play almost anywhere.
Headphone Amplifiers are most commonly deployed in home or project studios where several musicians may simultaneously need to wear headphones for live recording purposes.
Sophisticated Individual Sound Control
Instead of driving speakers the headphone amplifier drives several sets of headphones simultaneously and is sometimes referred to as Distribution Headphone Amplifiers. Users have control over individual balance and volume settings while on some of the more sophisticated models have loudness bass and treble controls.