It solves headaches that come from trying to project the not-flat earth onto flat charts. As Wikipedia puts it: "In most projections, scale varies with latitu

It solves headaches that come from trying to project the not-flat earth onto flat charts. As Wikipedia puts it: "In most projections, scale varies with latitude, so on small scale maps, covering large areas and a wide range of latitudes, the linear scale must show the scale for the range of latitudes covered by the map." "Mariners generally use the nautical mile, which, because a nautical mile is approximately equal to a minute of latitude, can be measured against the latitude scale at the sides of the chart."