Paraje San Diego was the final stop before the Joranda del Muerto, a 100 mile journey with no water source that needed to be completed in the shortest amount of time possible due to the dire circumstances. On the other end was Paraje Fra Cristobal. This paraje is considered significant for that reason.
If you'd like to learn more about El Camino Real, please visit my post on that titled Jornada del Muerto Historical Marker: Doña Ana County, there are some resources at the bottom of that posting.
There is also this brochure from the Camino Real Heritage Center.
Check out this marker mapped out HERE.
This last photo looks out east from the rest stop on I-25 where this marker is located. The view is of the beginnings of the Jornada del Muerto journey.