>>418616613
I wouldn't say immersion mainly; it's mostly to make sure you look how you think you look. 
In RL you have senses on which muscles you are activating and where your body is positioned but in VR you can't tell what facial gesture you're making. There is an immersion component; actually seeing your avatar move as you do links the funny brain cells into immersing you a bit more.
As for using the facial hud thing; I find that less immersive than an object in the world.