When you record a path for a vehicles autopilot, it creates a series of waypoints that the vehicle follows. In particular, the pause waypoint can be edited so you can change how long a vehicle stays parked before continuing on.
However, in order to toggle the visibility of these waypoints after the path is recorded, you have to actually be inside of the vehicle.
If you want to adjust how long a vehicle waits at a truck stop, for instance, you need to wait for the vehicle to come back around, quickly hop into it and toggle the waypoints on, then hop out and edit the desired point. Then, you need to get back into the vehicle again to hide them.
That's only one instance of this being useful. There are other applications for this as well. It would just generally be useful to have a quick toggle to automatically show or hide all waypoints without you having to actually be inside of the vehicle.