This isn't limited to just geeky t-shirts with obscure references. Clothing is one of the easiest ways for a person to indirectly advertise their interests. This helps lower the 'awkward approach' barrier for strangers that might like to strike up a conversation. Instant conversation starter.

Even living in the PNW, I still get plenty of waves and cries of "Go Pack!" whenever I wear a hat or shirt supporting the Packers. I wear the memorabilia for myself, but the social benefits don't go unnoticed.