[gonzus]

One of the advantages of doing this seemingly weird projection is that you can treat "local" maps (for some definition of local) as flat rectangular grids without introducing a lot of errors: drawing straight lines between two points, measuring the distance / angle between them, etc., just by dealing with a flat piece of paper. VERY convenient, but the farther you are from the center of the projection, the higher the errors that are introduced.

[Sharlin]

In short, you can treat the local geometry as Euclidean.

[bregma]

Or, to put it simply, the shape of the Earth can be considered flat for local mapping purposes.

If grid north and true north are the same everywhere, it would be proof the entire Earth is flat.