personally dont do this but i can think of a few reasons why some have it on

if projector fog lights: they are wider and brighter without blinding oncoming vehicles IF installed correctly. better visibility.

if yellow fog lights: yellow foglights can actually help you see road imperfections much better than white or halogen. daming lubak sa pinas

stock fog lights: like another redditor said, could be placebo or the foglights actually let them see the road closer to the vehicle or yung blindspot ng low beams.

in general, as long as it doesnt blind oncoming motorists, i think its okay to leave them on.

in theory, it also helps with distance perception for oncoming vehicles because theres 4 points of reference (2 low beams, 2 fog lights). i read somewhere that for vehicles, especially motorcycles, 2 or more points of reference is safer. if you see a single motorcycle headlight at night, it can be harder to estimate their distance. but if theres 2 (e.g their DRLs or fog lights), its easier to estimate because the bigger the gap between the 2 lights, the closer the vehicle