>>549355
I think it's a nice reminder of your faith. It's not needed, necessarily, but it also helps others to be more open about their faith. It's the same reason ancient Christians used to draw the fish on the ground - to know who was their brother. It would be nice if we had a greeting (like the Muslim "salaam alaikum"), but I doubt any of us could agree to any one thing.
These days, though, especially here in the US, it could just be some ironic hipster. They can usually be spotted because they wear actual rosaries as jewelry, but that's not always the case.