A friend of mine does that with her boyfriend as a sort of solidarity thing with same sex couples. Dunno if that's Aaron's reason, but that was what I thought of.
In some circles it's common to use the word in more formal or public settings when you are together for a long time (long enough that most couples would be married).
It seems to be what grown ups call their boy/girlfriends.
This, for me. Ironically, my "partner" regularly wonders why I don't call her my "girlfriend". Perhaps not coincidentally, she is quite a few years my junior.
At 40, I use girlfriend. Partner is a much more deliberately ambiguous word. Someone who I introduce as my partner might not even be romantically linked with me.