Most gates, compressors, limiters and some EQ's have a sidechain. It essentially works like a trigger. Whenever the sidechain receives a signal it activates the device.
You can use it to 'duck' a signal. For instance the kick could trigger a compressor on the bass so that the bass is turned down when the kick is hitting.
You could duck a reverb or delay track whenever the main dry track is playing.
You could use an undersnare mic to trigger a gate so that it opens the top snare mic or a room mic when the snare is struck. That way you don't have the high hat triggering the gate.
There are lots of weird uses when you think about it.