If you do positioning in degrees, minutes, and seconds (or decimal minutes which I guess is more common nowadays), a 'natural' length unit for navigation is a minute of longitude, which is the original definition of the nautical mile. And then we get velocity as nautical miles per hour, or knots.

Blame the French for reinventing the wheel instead of, say, using a 1/1000th of a nautical mile as the definition of the metre. Alas, that's all water under the bridge by now.

If you want to do navigation in SI units, the UTM projections (see sibling thread) typically do positioning in metres inside each grid square.

Because nautical miles is an angle measurement which then happens to be 1852 meters when you break it down.