[danisth]

You set up the base station over a known point, and now your rover unit can give you cm level accuracy when it's within range of the base station (UHF radio range generally).

For a practical application, let's say you wanted to lay out crops for a small hobby farm. This can make getting your base station set up, because you might not care as much about absolute location, which would require finding a surveyed benchmark. Instead you put your base exactly on the corner of your plot, and after setting up a local grid system you can now exactly layout your farm according to your plan. You simply walk around with the rover unit and can get real time cm accurate "coordinates."