Red mode

Human eyes take twenty to thirty minutes to fully dark-adapt, and one flash of white light resets it. Red light does not, because the rod cells responsible for night vision are barely sensitive to those wavelengths. Fifteen seconds of white light to tie a knot costs you half an hour of seeing in the dark.

It also matters for the fish. In clear, shallow water, white light shining onto the surface visibly moves fish out of casting range. Red light does not.

Some lamps offer green as well, which preserves night vision less than red but shows colors better, and both are quieter around the water than white. Green also shows blood and line more clearly than red does.

The other advantage is social. A headlamp swept across a boat or a bank in white light blinds everyone it passes, and red mode largely removes that problem without anyone having to ask.