It's another line of defence in a situation where there are aircraft holding at several different runway entry points. We all know of incidents where the wrong crew has taken a line-up clearance as they have missed or misheard the callsign being used. If the ATCO also specifies the runway entry point in the line-up instruction, there is another chance for a misheard instuction to be queried on the flightdeck, as the entry point won't tie up with where the aircraft is holding.