You're expected to voice chat in comp because if you're playing to win, you need to communicate with your team. You can't do this by typing because that means you've stopped playing in order to type it. So you need some way to communicate with your team without using your hands--and would you look at that, you have a mouth that you probably aren't using for anything else at the moment 
This is necessary because you need to tell your team if you, for example, notice an enemy going behind you or trying to flank, or telling people when to use their ults or what abilities are currently on cooldown so you can effectively combo.
This is necessary because you need to tell your team if you, for example, notice an enemy going behind you or trying to flank, or telling people when to use their ults or what abilities are currently on cooldown so you can effectively combo.