In practice, when navigating by pen and paper, what you do is you measure a distance on the chart with a divider, then you put the divider (with the previously measured distance set) to the north-south scale on the side of the chart to read out the distance in nautical miles.

Now, of course, in principle charts could have a separate scale for distances in km, and we could use km for navigation just fine. But, well, I've never seen a nautical chart with such a scale.

And, at sea level. At altitude this no longer applies either because the circumference at altitude is greater.