1. You won't look like everyone else. Back then, clothes were often custom-made or produced in small quantities sometimes with unusual fabrics that aren't manufactured anymore. Today, you can be on vacation at the other end of the world and run into someone wearing the same dress you bought at h&m. Especially, if you're a creative type or like to be noticed, vintage clothing offers a wide range of imaginative styles.
2. People of past generations didn't throw their stuff away as easily as we do today. Clothes were made to last - better material, generous seams for alterations - so they could be worn for years and handed down to other family members. A friend of mine still wears one of her granny's cardigans, it looks like new - unlike the expensive one I bought a few years go which frayed after the first wash!
3. Many of these old clothes have unusual details - pretty buttons, hand embroidery, lace, beading and other techniques for which you would pay a fortune on a modern garment.
4. Buying (or donating) old pieces contributes to sustaining the environment. The less people trash whatever they don't want, and instead recycle, reuse or repair old things, the less new stuff and its waste will be produced...
Vintage clothes are so much cheaper than modern ones, if not even free - maybe you can obtain a piece or two from an older friend or relative, and alter it to fit you. If your parents were young in the 60s and 70s like mine, your mother might have a mini or indian hippie dress or two stashed away in a trunk in the attic!