To have some light if a headlamp goes out. Safety. No longer a motorcycle coming at you, but a wide vehicle. But, they are still used as parking lights when you want them to, like parked on a side street. Not needed when moving.
Now three pages later, the real reason NOT to use "parking or marker" lights when driving is when it gets darker the driver will forget just the parking lights are on. I see this all the time in city and suburban driving. Only when the car gets out in the country does the driver notice he cannot see. Sure, you think, I'll remember, then the kid cries, the wife gets you in conversation, you yell at other drivers for flashing their lights at you (what's wrong with those people?) and finally, hey why can't I see the road, my lights are on, just look at the dash.....😮