Audio Plugin Deals has published an article that breaks down the two fundamental uses of side chain processing.
Side chain is both music production and mixing stages is a useful tool.
It can be used for a stylistic purpose such as pumping the bass and add new rhythmic elements to a section or all of the additional instruments. And it can be used more subtlety to help balance out key elements of your track and achieve a more coherent mix overall.
More information: Audio Plugin Deals