Unlike the 'Designated Driver' they are allowed to be intoxicated, but not to the point that they can not think rationally enough to look after their fellow revelers.
This may include, making sure all of a group get to the next destination, stay off roads, don't eat dodgy hot dogs, kebabs, etc. They may also end up having the responsibility of soothing the individual lying in a pool of their own vomit or tears....
The designated drover can be determined prior to the commencement of an outing or assumed at some point in an unfolding crisis.
You were lucky there was a designated drover, otherwise:
you'd have been hit by a car for sure.
you'd have gone home with that crazy_____ for sure.
you would have had vomit in your hair for sure.