It can't just be me who doesn't get it; have seen threads where people use 3/4 length sleeves as reasons for deciding against something.
Surely if it's cooler you want proper long sleeves (3/4 knits are especially strange) and if it's warmer, 3/4 is too warm? I can see some uses for work where you need to frequently look at a watch or roll up your sleeves a bit, but that doesn't account for their popularity.
I've heard they are popular in plus size ranges because some people really want to hide their upper arms - but there are lots of them available in small sizes too.
Do some people's forearms just not get that cold?
(I can understand people wearing short skirts in cold weather because I have some of that cold-tolerance in my legs, and have read that fat distribution in the legs helps with the physiology of it, but arms, no - never developed it.)