I keep mine on for a couple of reasons-

1 - if I don't I'm pretty much guaranteed to not have it when I need it. (shooting towards a rising or setting sun) Had to do it multiple times at PRS matches and a once or twice at a couple of the CD matches.

2 - when shooting on a troop line (multiple shooters all in the prone next to each other) it helps to keep some of the dust/dirt off the objective lens from the heathens that run brakes

3 - when shooting in rain and snow, it helps keep moisture off the objective lens.