[kittonian]

Stereo compression would be used on such things as drums (be it overheads, room mics, or the entire stereo drum bus either in line or in parallel), stereo instruments such as piano, a stereo group of backing vocals, or the entire mix to give it some glue/punch.

There are certainly stereo compressors that are better chosen for certain tasks. For example, the Chandler TG1 works amazingly well on the drum bus. The DBX 160SL works incredibly well on piano and the SSL G Compressor works fantastically on the mix bus. Certainly these are my opinions and there are a wide variety of compressors to choose from. Each has its own sonic characteristics and will perform well on various sources.