When taking out our trail strings, we also call the end rider a 'drag rider'. I often do that on large groups as I consider the drag riders to have a much more important job than the guide. Guides do not have eyes in the back of their heads and need to watch the trail ahead.
The drag rider is responsible for seeing everything in front of them and telling the guide to stop so the drag rider can make adjustments adjustments and fix any problems. The drag rider has to make sure we don't have someone using the reins for a '3rd stirrup' or have a horse that is getting crabby with the horse behind them (read horses well), watch for riders that are tormenting their horse, pick up any items dropped by a rider, and the list goes on and on. Riding drag is not just a person going along for the ride. It is the person that makes the ride go off without a hitch.