The USNG is a single grid referencing system for the contiguous 48 states. Here’s what the folks in charge of its implementation have to say about it.
“USNG provides a seamless plane coordinate system across jurisdictional boundaries and map scales; it enables precise position referencing with GPS, web map portals, and hardcopy maps. Unlike latitude and longitude, the USNG is simple enough that it can be taught and effectively used at the 5th grade level. It enables a practical system of geoaddresses and the universal map index.“
We find some of the claims to be wildly optimistic, especially the part about teaching the USNG to 5th graders.