It's been required to do so in some northern European countries. The practical reason there is that dawn and dusk can take many hours during much of the year at

It's been required to do so in some northern European countries. The practical reason there is that dawn and dusk can take many hours during much of the year at higher latitudes thus creating a weird situation where it's not really clear when exactly you should have the lights on or off in exactly the kind of situation where a lot of accidents can happen if you don't have them on. The simple rule of "always have your lights on" fixes that and it slightly improves visibility even in broad daylight. The only downside would be the increased power usage and the light bulb wearing out sooner. A minor issue a few decades ago when this became normal and a very minor one now that we have LEDs that last a long time without using a lot of power. My rental bike (a Swapfiets) has the LEDs permanently on; there is no off switch. If you ride it, the lights are on. That's indeed how it should be. I can't think of any good reason to voluntarily reduce my visibility while cycling through potentially lethal traffic.