I guess it also stops people from leaving. Since once you made an order you're more invested to stay and pay for it. But if you were in a long line you might get fed up and go somewhere else (or get a phonecall to do something etc).
True, but I've generally found the line moves fast even when the line is out the door. The main bottleneck in most fast food places are putting your order in at the cash register so if that's removed it speeds things up a lot.
