So others can pass without waiting for a normal passing zone.
If you’re driving in that part of the country and someone starts riding the shoulder they are probably trying to signal for you to do the same. Unlike the area where I live, those roads have shoulders that are almost a full lane width and are paved the same as the regular lanes.
It may seem unusual, but it’s considered common courtesy in such areas to ride the shoulder to allow the vehicle behind to pass. My guess is this came about because traveling the desert roads often involves a lot of distance – nobody wants to be stuck behind a slower vehicle for a long time. The availability of safe passing areas without riding the shoulder varies.
A common signal that someone wants you to drive on the shoulder is simply that they’re riding close on your rear. Basically, any time a vehicle is moving as though it’s ready to pass is usually a clear indicator.
It should go without saying that common sense applies when performing this maneuver. For example, in any place where the shoulder narrows or there are guardrails are bad locations. In addition, it’s wise to make sure that the shoulder itself is clear of obstructions. And, as usual, if it seems risky at the speed limit then just let off the gas a bit once you’re over.
While I don’t know if this is officially sanctioned, I can testify that I witnessed a highway patrol vehicle do this very thing in order to allow us to pass, while he slowed down for unknown reasons.