Being a kid from the 80´s, I still remember some of the 90´s fashion and the coolest one was definitely the button-down shirt tied around the waist.
Purpose? Style points. And that´s it. None other, really.
Some pointers on how to do it:
1. Tie it low around your hips.
2. An oversized shirt looks better.
3. Plaid is cool, but feel free to use other fabrics.
Oh, I think you noted that´s it´s (again) an unisex trend.