Conditions for identifying aircraft are laid down for Air Traffic Controllers in the 'Manual of Air Traffic Control'
When I was controlling it was necessary to ask for an 'ident' when the selected squawk had not been verified by the initial unit handling the aircraft. For example, if a squawk had been given to an aircraft departing an airfield that didn't have SSR and couldn't ensure that the transponder was correctly set. It was the only way I could positively identify i was talking to the aircraft I was expecting, and that someone else had not accidentally selected the same squawk.