I had one because I had no ID on me that gave proof as to my identity.
Car was taxed/insured, and the name (mine) I gave them came up clear. But silly me left my wallet at home, and all I had was my mobile, add that I happened to be wearing an old Corperate fleece advertising my mate's business (which unfortunately for me was named after himself).
So I had a wee bit of difficulty getting them to belive I was who they said I was
PITA though - I didn't even get any form of reciept at the police station
I had to ask to write me one as I didn't trust them.