Prevents you from accidentally solo-queuing a Battleground when in a group. Disables the Battleground solo-queue button in groups, so you can't click it. Long-term, this will save you a lot of embarrassment and inconvenience!
Hold down Alt when you're sure you want to solo-queue anyway. The button will light back up.
Tiny, and very resource-conscious; does nothing except when the PvP queue frame is open, and even then does almost nothing.