This aviation microphone cover is slipped over the foam microphone cap but can also replace the original cap when it is missing, torn, worn or dirty. (Type Micro Electret parallelepiped)

Its usefulness is first of all a means of hygiene, but like the foam windscreen, it reduces the surrounding noise and prevents the air pressure from activating the microphone. You will set the squelch to a lower level.

The VOX "Voice Activation", which is the level of adjustment to allow voice activation, will also be set lower.

 But one thing you are never told...

This cap protects not only the foam cap but also your microphone. On modern headsets, the microphone is very often electret... The principle of the electret is the small membrane of the microphone which is stretched by a small electronic circuit in order to cancel the ambient noise of the cockpit captured. This is done on both sides of the microphone.

This is also the reason why you find "Noise Cancelling Microphone" which should not be confused with the ANR principle. The principle here is not to create an inverse wave to the perceived noise but to cancel a residual vibration which makes the membrane of the microphone vibrate.

Electret microphones are very fragile, the membrane is a very thin and flexible plastic film (a bit like cellophane film) which makes it afraid of humidity and cold. The overcap or the foam protection cap will act as a barrier and will filter the humidity coming out of your mouth, either the overcap alone acting as a "foam cap" or the cap alone or with an overcap are essential.