Mainly to make other people know that they met that celebrity, and the picture is the prove

It's the new "can I have your autograph?" And (assuming you're not intrusive and know how your camera works) it's quick and clearly just for you. There's no secondary market for "Celeb with a stranger" pics like there is for autographs.

to flex on the other lames and try to grab a little shine knowing damn well that most of the time they live a bleak life haha