Noise gates serve to lessen unwanted noises, like hisses and hums.
Adding a noise gate to your rig can drastically decrease any unwanted humming, hissing, or string buzz in your signal. Especially for metal players, who are prone to such noises because of the high-gain settings of their pedals and amplifiers, noise gates can help eliminate those undesirable sounds while leaving the rest of your signal unaffected.
Using a noise gate, you can dial in settings that make quieter noises under a certain threshold even quieter. So if you’re experiencing some unwanted hissing, you can use a noise gate to find the threshold where that noise becomes bothersome and adjust your settings to quiet everything below that, while leaving your good signal intact. A pedal used for a rig-wide hissing issue can sit at the end of your chain.
Alternatively, you can pair a noise gate with a specifically noisy fuzz, distortion, or any other effect that gives you problems and set the gate to combat the noise from that problem pedal.