Here are some examples of where you can use sidechain compression in a mix:
Kick & Bass - ducking the bass out of the way of the kick to create low end space
Pumping effect - for EDM, RnB, Hip Hop & other genres
Vocals and Instruments - ducking the instrumentation out of the vocals creating more space
Vocals and Reverb - providing more clarity for the Vocals
Snare & entire mix - gives a short pocket of space for the Snare to cut through
Creating space in specific frequency ranges - using multiband/dynamic resonance suppressors to sidechain problem frequencies
Volume automation - creating the sidechain effect without compression, often using a sidechain plugin