It is perhaps seen as proof that you really have met the person in question in order
to avoid people saying 'No you haven't'.

Nowadays a photo taken with a mobile phone is probably a better choice, and harder to argue with for the non-believers.
(thediscokingpin, Tue 7 Jul 2009, 10:32, archived)

I don't know why it even matters,
although my dad's got a photo of my brother with Rolf Harris, and I felt the need to mention that just now, so... I still don't get it.
( Moon Girl Technologies horrendous beanbag, Tue 7 Jul 2009, 10:38, archived)

I think you are either impressed by it or not,
but i'd say it was a sign of a more involved and perhaps interesting life if you recognise and have a chance to say hello to people who do something that is meaningfull to you.
(thediscokingpin, Tue 7 Jul 2009, 10:44, archived)