The most practical reason is as a guideline to avoid distortion and speaker damage due to clipping by running an amp with too little power too loudly.
In general, solid state amplifiers that deliver more current (amps) are able to better drive most dynamic speaker designs with varying impedance curves at various frequencies (referred to as a "difficult" load)and deliver more balanced sound from low to high frequencies.
In general, solid state amplifiers that deliver more current (amps) are able to better drive most dynamic speaker designs with varying impedance curves at various frequencies (referred to as a "difficult" load)and deliver more balanced sound from low to high frequencies.