If you're playing a team-based game and you want to communicate with your teammates so that you can coordinate teamwork, it's easier to do with a mic, especially for a first-person shooter.
Consider a 5v5 game of capture-the-flag where two captains draft-picked the teams (like in gym class) and everyone has assigned positions (defense, front door defense, offense, etc.). You would want to use your mic so that you don't have to stop in the middle of playing to type out the communication and communicating quickly is of the essence.
"Flag carrier going low."
"He's in his base."
"There's 3 guys waiting for you in our base."
"Take it high, I'm waiting to cover you."
As sophomoric as it might seem, a game like that feels like you're playing an athletic cybersport.