When I first moved here for college at 18, I was skeptical. Really, I thought they were playing a joke on me. People pin money to themselves? Moreover, strangers give you money? Yeah, sure. After all, my whole family is from New Orleans, and I'd visited literally dozens of times, and yet I'd never heard of this supposed "tradition." In my very highstrung brain, addled by going to school outside of D.C., I knew they must just be messing with me.
Well, live and learn. It is in fact a tradition, and it's one that to me really embodies the spirit of the people of New Orleans. People want to know that it's your birthday! They want to holler "HAPPY BIRTHDAY" at the top of their lungs across streets and in the middle of busy stores. They are so happy that you've allowed them to share this day with you that they add a dollar to your b-day collection. I understand that this occurs a few other places around the country... but very few.
I know it sounds strange. I've been participating for over a decade now and I still don't believe this is how it works. But it's for real, and it's wonderful, and I personally will be pinning dollars to birthday boys and girls till the day I die.