Noise gates can be actuated by the input signal itself, OR by something else entirely different. That something different is the "key in".
So what good is it? Two examples:
1) If you listen to the David Bowie tune "Let's Dance", the horns sound sorta like horns, but sorta not. During production, producer Nile Rogers run the mixed-down horns through a gate, and used his guitar strumming to gate the horns in guitar-like fashion.
2) If a person wanted to stick a gate at the very end of the signal chain where the most noise had accumulated, but use the envelope of the original input signal, where the dynamics had the greatest contrast, they could run the guitar into a splitter. One output from the splitter would go to the rest of the pedal chain, and the other would go to the key in, so that the original guitar dynamics could be used for controlling the gate.
