Answer: because they like it that way.
No there is no law AFAIK. But last I heard, you are encouraged to do so as it increases visibility. Not for the driver, of course, but to other road users. Over short distance it doesn't make much of a difference, but oer long it does.
This of course excludes mandatory usage in tunnels etc.
I've had to endure years of my father commenting "you do know your lights are on..." on visits.
http://www.admin.ch/ch/d/sr/741_11/a31.html
5 Die Abblendlichter oder die Tagfahrlichter sollen bei Motorfahrzeugen auch tagsüber eingeschaltet sein. 3
Also in many EU countries, it is mandatory all year around, and some only in the winter months.
Plus:
As a result of European Union Directive 2008/89/EC coming into force on the 7 of February 2011 all new passenger cars and small delivery vans through out the EU will have to come equipped with daytime running lights.
http://en.wikipedia.org/wiki/Daytime_running_lamp
Looks like you will see it more and more.
I want to say that "the majority" of new cars in the US tend to have "Daytime Running Lights" as part of their default package these days, specifically because it does help improve how well they're seen by others.
Also though, there are places I've lived where it is the law to have lights on when it's raining and such, having them on all the time helps mitigate the "doh!" factor of forgetting to turn them on in the on-again-off-again spring showers.
Lights serve 2 main purposes:
- to illuminate the road (the way ahead) in dark conditions;
- and to help other road users see you (your vehicle).
Put simply .... "see and be seen".
If looking (roughly) towards the sun at a light coloured car, in bright conditions; or at a dark coloured car on a wooded, shady stretch of road, the car is easier to see if lights are on.
As a motorcyclist I have always used a headlight during all daylight conditions -- in fact on my current bike there is NO switch to turn the light off - it is permanently on.
I have also driven in Sweden, where it is law, and that is why Saab's and Volvo's, are always manufactured to have the corner lights permanently on.
Remember it costs you nothing to switch them on, but could cost dearly not to.
Or more likely they had been through a tunnel and couldn't be bothered to turn them off!
If I have any light flashing wildly red, I normally spot it in my peripheral vision.
Watch out for "stunned rabbit syndrome" (similar to "lamping") - I've had motorists look directly at me (and my light) and still attempt to drive at me/into my path from side road junctions.
This is because (I believe), that when a "light" is steadily approaching in a straight line, with no lateral movement relative to them, that they end up seeing through and beyond the light, without it registering it in their mind as another vehicle. If/when I see them trying to pull out, I just wiggle/weave from side to side a bit, and they usually fully realise my presence and stop their manoeuvre - but not always guaranteed.
On a motorbike, you shouldn't flash (high beam) them, as this could confuse and lead them to believe they can go. However, speaking personally, I have noticed, in dark conditions, that I notice pedal cycles quicker and easier if their front and back lights are of the rapid blinking variety.
However, the Jeep Cherokee we drive in the USA has exactly the same problem as PaddyG's. You can't see the clock, gas gauge and quite a few other displays at daytime with the lights on, because they are automatically dimmed. I still haven't found a way to prevent that. The car was made in those years when no one cared about headlights in daylight. The manual gives no clues. I'm afraid we would have to modify the wiring.
P.S.: Of course we do have a Heads Up Display, but we usually keep it switched off because it interferes with the periscope for the torpedo launcher.
NOT I might add the 'foggies' as most people seem to do here as well !
I am personally in favour of cars having the lights on in bad light, but not really during sunshine