When I was a kid my mum used to make me tie my sweatshirt around my waist to prevent me from losing it (the consequences of losing anything when I was young were severe!!). I hated it so much. I thought it was so uncool that I would swelter for as long as possible with my sweatshirt on just so I wouldn’t have to tie it.
Today, however, the sweatshirt tie has become somewhat of an accessory. I say 'accessory' because if you look at a lot of the street style snaps of people invoking this trend you can tell quite clearly that the intention with the sweatshirt around the waist is not usually for ease of carry (or to prevent loss). They are not intending to ever actually put what is around their waist on, nor did they ever have it on. The intention is purely for looks – Like wearing a necklace or bracelet. The great thing about this 'accessory' is that you don’t actually have to buy anything new to work it. Just grab any long-sleeve top/jacket/shirt and you’re in (Christine Centenera even uses a blazer!).
Images: theyallhateus, @jesskent91, Shine by Three, Harper & Harley, Google Images.