The reason is there is a law. I turn mine on when I hear a boat coming. The light attracts insects. I mostly fish lighted boat docks and marinas. One nite I was in a Marina sitting pretty much under a street light. Some idiot in a sailboat started raising hell because my lights weren't on. Ahoy mate, where are your running lights?. I yelled back, well you know.
Perch, I came close to running a guy over on the river last night. It was a dark night and he flashed a light when I was closer than I would have liked to be. I wasn't going that fast but I sometimes run fast through there and it is a regular area where boats go to avoid rocks in the middle. He was sitting still in total darkness.
As I went by I thought I was being friendly and said "You need to turn your navigation lights on buddy." In my mind I was thinking I might save him being run over. I heard him smart something off about minding my own business,but I never said anything else or slowed down.
Later I was thinking there might be some technique where total darkness was needed bass fishing I didn't know about. I thought you had to have your navigation lights on anyway, but then I got to thinking I might be wrong and they might only have to be on if the motor is running.
Nevertheless,I thought it would have been a good idea to have a light on since I've seen guys who would have run over the guy if he did them the same way he did me because I've seen them running through there a lot faster.