It’s a form of styling and layering, so you look like the cool kid on the block. The trick is to wear the jacket, whichever kind it is, halfway to your shoulders so your look like you’re about to take it off or just about to put it on. Hence, the jacket shrug.
If you aren’t yet convinced, check out some of the pictures below of popular instagram stars styling their jackets this way. Remember this type of styling is more suitable for casualwear.