A small amount of UV exposure is necessary for some processes, including Vitamin D production and a few others.

The tricky part is that it is very widely known how dangerous UV is, and people with an outdoor lifestyle can easily overdo it, but at the same time there is an increasing portion of the population that spends almost ALL time indoors (except when they are on vacation) and gets close to NO UV exposure normally.

This almost total lack of exposure to UV can be especially problematic due to high latitude, winter, or even slightly high latitude and darker skin tones.