Very useful for when you make rooms with GTP in the title to organize a race here on the forum. Whether people wander in or not, when the GTP event is over... we don't want to leave the room lingering for people to mess around in. Especially now with all the people doing the SSR7 thing, if a GTP race ended and some non-member was left in charge of the room and started doing the money hack with a bunch of friends and then someone else joins the GTP titled room expecting to find good racers but instead finds a bunch of cheaters... we ALL lose respect and this great forum will start to become associated with that kind of people.
On the other hand, leaving it unchecked means that if you're hosting a room (say maybe during a very important series race) and your power goes out suddenly or your internet dies, the room stays open and everyone can continue to race. You wouldn't get to finish the race and may drop in the series point standing, but it's better than everyone getting booted. Then you have 15 angry people and then you have to figure out if you're going to drop the race from the series or redo it or whatever.