The Key input is like a control line for the gate. It allows you to gate a heavily distorted signal with the clean signal directly from your guitar.
Ever put a bunch of dirt / fuzz / whatever in front of a gate? The signal is so hot (and noisy) that the gate doesn't react properly. Breathe the wrong way and the white noise from the pedal chain will open the gate, defeating the whole purpose.
Sure, you could put the gate BEFORE all your gain, but if you do that it won't silence the noise floor coming from the high gain pedals...
The Key alleviates both problems by controlling the gate with a pure clean signal before it gets hammered by the preamp / pedal chain, but gates the signal post-gain so you actually get silence when gated.