[flexoffset]

Side chaining is useful in 'ducking'.
For example, you have a bass guitar that hits the same time as a kick drum, let's say.
With side-chain compression, you can use the audio from the kick drum to trigger the compressor to suck down the bass guitar and lower the level of the bass guitar for the duration of the kick.

You can also use it creatively to de-ess vocals, hats, etc.
You can use sidechaining to trigger gates, etc, too.