If a group needs to split up for some reason (mechanical issue, some stay back) it's fairly common that the tail gunner becomes the new leader for one of the groups. Not always, and not necessary which one, that's up to the leader/gunner to decide... but it's why the tail gunner should generally have a pretty good idea of the trail, and how to get out of there.
Also, for trails with one or two "big" obstacles, where everyone on the run pretty much watches everyone on the run, the tail gunner can often be the best designated spotter. That allows the leader to move up once a good portion of the group has surpassed said obstacle, and get the group started on the next one. Great example is Kane Creek- tail gunner stays back to spot and makes sure everyone is past the shelf road, while the leader gets the first few rigs queued up for the waterfall at the end. And since later in the day there are more competing Jeeps, quads, and motorcycles.. briefly splitting up into two groups helps manage that a little better as well.