Most gates cut off the hum and buzz when you aren't playing, anything from stopping feedback to hard gating in short palm muted parts.
Some of the causes of hum are high gain and cheap components, but also the are unshielded cables, ground loops, interference picked up by racks, noisy guitars and other components.
To really quiet the noise every aspect of the amp, effects, cables and guitar should be investigated. A noise gate can be a bandaid on other problems in a noisy setup. In an optimized rig though it can be used much more sparingly and still help.
I am building a setup now with an isolated power distribution, balanced send and return cables, wireless inputs, very short cable connections, rack isolator tabs and quiet humbucking guitars. It still has a buzz on the distortion channel but that's what the gate is for.