1. It's ethical. Whether you scour thrift stores for diamonds in the rough or browse etsy for a more refined vintage shopping experience, buying vintage means that you're not supporting big companies that outsource their labor to factories in poor countries with bad working conditions. You're not sending your money to a corporation that underpays its employees, uses sub-par materials, or exploits its workers.
2. It's green. When you buy used clothing, you're preventing good clothing from going to waste. You're also not taking part in the phenomenon known as "Fast Fashion" - where you buy something that is more or less disposable that you only plan on wearing for a season.
3. It's interesting. You're not going to walk down the street and find someone wearing the exact same outfit as you, and that is a good thing. You'll stand out in a crowd in the best possible way.
4. It's well made. If an item has made it this long without falling apart, it was probably crafted with some skill. You will find that many vintage items were either handmade and/or made in the USA. You won't get that with clothing from most low to mid-range priced stores. Even higher priced items are often made in China.
5. It can be very inexpensive. If you're willing to dig through piles and racks at the thrift store, you can get beautiful garments for pennies on the dollar. If you're not, you can get plenty of really great clothes on etsy for the same price as Forever 21. No, really. You can find plenty of dresses in the $20-30 price range, and you can wear them for way more than a season.
6. It's really fun. It is sort of like playing dress up, except everyone will think you're unbelievably cool.