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."

On earth, the meridian has an approximate length of 20,003.93m.

    20,003.93km / 180 / 60 ≈ 1,852.22m ≈ 1,852m = 1NM

Note, however, that

    20,003.93km / 200 / 100 ≈ 1,000.20m ≈ 1,000m = 1km

so if we used gradians/gon where the full turn corresponds to 400 instead of 360, the kilometer would be an approximation as good as the (international) nautical mile.