More like an Autumn/Spring thing than a Summer one, but…

If you live in a place where temperature varies a lot across the day, you’ll want to wear a jacket at some hours, but not others. Then you need to choose between three options:

1. Wear the jacket through the day. You’ll feel hot at ~noon.
2. Don’t bother with the jacket. You’ll feel cold at ~early morning and ~early night.
3. Take off the jacket at ~noon. Now you’re carrying (and potentially forgetting) yet another item.

All three suck. But people disagree which one sucks the least, and for some it’s #1. So you get people wearing jackets even when it’s too hot for that.