Originally Posted by Justiciar
Why actually do we use knots?
For navigation purposes. You're right, it is not very relevant when flying short distances but that is the reason. Back in the pre-WWII days a DC-3 might have an ASI in MPH on the left (captain's) side of the cockpit as he did the flying, and an ASI in KT on the right (co-pilot's) side of the cockpit as he did the navigating. This made for confusing situations when changing seats.