Maybe to prevent two controllers 'scottish or London' doing the same thing at the same time & mixing what they see on the screen up.
I was thinking along the same lines, but I would assume that an ATC unit would only ask an aircraft to squawk ident if that aircraft is, or is believed to be, in that ATCs area of responsibility. So the lateral distance between the two and an airspace boundary inbetween would give controllers a clue as to who's who.