[Flex247A]

By compression they mean dynamic range compression, which loudens softer sounds so they are more audible.

Pop songs for example, are heavily compressed because "compressed" music sounds "better" on cheap external speakers with bad dynamic range. Kinda like increasing the saturation on a cheap TFT panel.

[LinAGKar]

It's also useful for cars, which I think is where most radio listening is done, since you may not hear the quiet parts.