Headroom: The Secret of tone? | Low vs High Power Amplifiers

not to be more loud, but to be more clean…

Headroom is an amplifier’s ability to amplify loud peaks in your signal without incurring power amp distortion effects. Pushing an amplifier towards its full power reduces the available headroom, causing saturation and sag from the power valves and transformers to start to take effect.

These effects may or may not be desirable depending on your application.
For many guitar genres being able to defeat your amp’s headroom is the key to unlocking the best sounds from the amplifier, so low wattage amps might just be your best friend.
PA amplifiers require a lot more power to achieve massive headroom so not to distort peaky instruments like drums and vocals.

Although your amplifier will likely be running far less than 20W on average, having more power is all about headroom: it’s to be cleaner, not louder.