system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does boats measure speed in knots and not mph/kmh?
gpt
Earth is measured that way (nautical miles). Each degree of latitude is 60 nautical miles. Each second is 1 nautical mile. So the earth is 360 x 60 degrees (or nautical miles). 

Therefore speed matches that unit for navigation purposes.  Also, it becomes an important factor when choosing map projection styles (converting a globe to a 2D map)...but that is a tangent.