The nautical mile is the standard unit of measurement
on a chart. Every minute of latitude equals a nautical mile- (not
longitude, for reasons that are apparent when considering a globe). Of
course we are all aware that a knot equals 1nm/hr.

Sailing vessels and aircraft measure speed in knots rather than mph. If
you were sailing or flying and you wanted to consider the effect of a
certain wind on your course or speed, it would be needlessly awkward to
convert between mph and knots.

Vessel speeds are more commonly measured in mph
on inland lakes or other waters which may be uncharted. Certain
motorboats frequently measure speed in MPH, offshore racers and
hydroplanes being
common examples. Take my trawler, for example.
I always consider my speed as a certain number of knots. Usually
somewhere in the 8-9 knot range. It would be silly to talk about a
trawler doing "10 miles an hour", especially since among other
inconveneinces one would have to switch to the statute mile scale on a
chart to time,speed, distance calculations. What a pain. On the other
hand, a boater with a speedy planing hull doing 35 mph or about 30 kt,
can use mph or knots almost interchangeably and both would be
considered acceptable for purposes of general discussion.