Pilot's charts show different details, but the Nautical Mile remains the best possible unit of measurement. As with many units, one must look at the people who use them the most to find out how they could be useful. The knot is another unit that pilots and captians like. It is a unit of velocity that is derived from the Nautical Mile. It would not be impossible to navigate the globe without Nautical Miles and knots, but these units make it just a little more convenient.
Convenience is the reason for the many different sets of units.