Where the medical establishment pushes it, it is largely about establishing habits for people who may get a lot of exposure that they didn't plan for.

Also, local climates differ dramatically. A couple of my worst sunburns in my life were on extremely gray days on the California coast. It is easy to fool yourself into thinking the sun isn't strong when you don't feel the heat of it. But on those kinds of days, it's just diffuse UV blasting from every direction.

I found it instructive when I got photochromatic eye glasses. Since they are UV-activated, it is like carrying a UV detection instrument around with a heads-up display. It really helped me get a better sense for what conditions and hours of the day have significant UV in my normal daily life.