Compressors DISTORT the acoustic volume RANGE of a sound source. In other words, at a certain sound level, it changes the rate of increase of the sound level to a lower rate.

Expanders do the reverse. They INCREASE the RANGE of the volume of the sound.

Compressors are useful for generating Muzak type sounds, i.e. of limited acoustic range, suitable for playing pseudo-celtic music in elevators so that people remain calm while plunging to their deaths when the cable snaps.