The Sid might have a final cleared level of say 8000ft but an initially you have to stop at 4000ft until passing a specified point before continuing the climb. If the 8000ft was set in the FCU and climb mode used the aircraft would automatically make the constraint at 4000 and then continue the climb to 8000. However if 4000 is set in the FCU then the pilot will have to initiate the remaining climb to 8000. It’s a more belt and braces way of avoiding a level bust, because if another mode is engaged for example vs then the 4000 constraint will be missed! Simple really.