When VR headsets are exposed to user space, they appear as displays and subsequently the display server will control them (which isn't useful), this is just a protocol that allows clients (like games, SteamVR) to have control transferred so they can drive the VR headset instead. This is because multiple applications are not allowed to control the same display on the Linux kernel at the same time.

It does not make DRM/DRI part of Wayland. Again, it goes back to my original comment of "you use Wayland to communicate with the display server"

IIRC GNOME originally wanted to do this over Dbus, but there was opposition.