Pin collecting and trading was a big thing for middle-aged women in the late 90s and early 2000s. It's making some sort of comeback with Disney I believe. Back in the day I remember the Padres actually having a kiosk at the stadium that only sold pins. It wasn't too uncommon to see someone at the game with a floppy hat completely filled up with pins.
Pins also made for a cheap and quick Frequent Friar reward. I've said it a million times, the Frequent Friar program needs to come back. I really had a lot of fun getting pins and baseballs and posters and whatnot.
As you can see, this pin is from 2001 and celebrates the Padres having attendance figures over 2 million for 5 years in a row. The team would continue with hitting the 2 million mark for an additional 8 years (including reaching the 3 million in 2004).
The pin is made by Pro Specialties Group which is a San Diego company that produces a variety of licensed sports merchandise.