3/4 sleeve jerseys hit about mid-forearm, giving you a middle ground between short and long sleeves. They're a niche choice, but they shine in mild conditions where you want a bit more coverage than a short sleeve offers but don't need the full warmth of long sleeves.
Mountain bikers tend to gravitate toward 3/4 sleeves because the extra forearm coverage offers light protection from brush and scratches while still keeping you ventilated on climbs. They're also popular with riders who simply prefer the look.
The trade-off is that you lose some of the versatility you'd get from a short sleeve jersey paired with arm warmers, which lets you add or remove coverage on the fly. If you ride mostly road, a short sleeve jersey is still your best starting point — but 3/4 sleeves can be a fun addition for those in-between days.