The edges of a six-mile hex are 3.46 miles, almost exactly equal to three nautical miles (3 x 1.151 = 3.452 miles), or one nautical league. This makes it easy to count columns sideways and report the horizontal distance in leagues (each column averages 1.5 leagues), just like it's easy to count rows vertically to get a distance in miles. (Assuming the orientation of hexes you show here.)
Wow, I had no idea that the conversion to leagues worked out so neatly! Thanks for this info, Edward.