Skin cancer.

And for the vain, aging - wrinkles, spots, dryness, etc.

I have fair skin in New England, use facial spf 35 sunscreen daily from May-September, and use spf 70 whenever I'm outside for longer than 15+ minutes. It's important as skin cancer is just bound to happen if you don't use it, as a fair skinned person. Not doing so would mean pieces of face/ear expensively sliced off when you're older. Both my parents grew up w/o sunscreen and got intensive burns, both of them got skin cancer on their face.

But even before old age and cancer, your skin will be healthier and look younger for much longer if you use sunscreen daily, use a hat, avoid 10am-2pm exposure, etc. The last one is interesting because during midday, the sun is directly overhead, meaning there is much less atmosphere between you and it, so even less UV is blocked.

My mom had skin cancer on her forehead - it was removed, and she's fine. Plus I'm bald. Like shaved-head bald since the age of 18 (mid-thirties now) - I feel like I'm more cognizant of the visible skin damage on older bald people's heads, and wow it looks bad, from both a health and appearance standpoint. Hence UV damage being top of mind for me.