In modern multi tasking cooperative work environments being able to mute or unmute your microphone without having to change the active window is a must.
It reduces the strain of having to look for the teams app while presenting to or working with people.
There is going to be an increasing number of users cooperating and this convenience feature is used by a vast amount of proficient users, gamers and ex-gamers that are used to the global hotkeys from TeamSpeak.
The use-case was the same, you were busy in an interactive real-time game, talking to people, and you didn't want to disturb your team with unwanted noises.
Having to ALT-TAB out of the game, or an application while working or teaching is much less convenient than just pressing a key anywhere any time.
Also, a global hotkey should be freely configurable not hardcoded and immutable. Since global hotkeys could easily clash with other applications, the purpose is giving the user to freedom to have their limited number but essential hotkeys in their environment.