Whether it's actually useful in full daylight (unless very cloudy) is questionable, but there are some countries where not only it is legal, it is mandatory.
It can be simply tested whilst on a country road during a sunny day.
I found no difference in visibility between vehicles with their lights on or off.
Whether it is any more effective in heavy traffic I don't know, but I doubt it.
The original idea came from Scandinavian countries where it is dark for most winter days.
You don't have bad weather? Running lights are pretty much irrelevant on a bright sunny day out in the open. Even on one of those days you only need to go under a big underpass, and the lights start to be effective. In bad weather they are a real boon.
If we have bad weather, it is
"slam you up against the wall & beat you up" type of weather.
Most of the rest of the time, it is reasonably sunny between showers, or even normal storms.
Those very dark, overcast days, where it looks like late afternoon all day are not near as common as in UK winters.
When visibility is bad, people
will turn on their proper headlights.
"Underpass"? ---We turn our lights on if we go through a tunnel, but an underpass would have to be seriously big to need to do so.