1. Not all bridges go to a single aircraft, some can be configured in such a way that a passenger taking a wrong turn could conceivably go to a different aircraft, which is why they need to check not only before you go through the gate but on boarding the aircraft
2. Not all boarding is done from bridges, some are done from Tarmacs, 2 aircraft parked in the same vicinity, passenger has to choose left or right and the attendants on the ground are distracted for that split second where they choose completely the wrong aircraft and wander off towards it!!
(Plenty of examples of 1 and 2 happening, there was one a while back about an entire aircraft of people walking through a number of closed barriers and making their way onto an aircraft that was completely dark with no one on board at all!!)
3. To check where you are sitting and direct you appropriately
4. As a way of greeting a passenger in a warm and personal manner by using their name
5. SOPs, not all bridges are the same and go to others, not all aircraft have more than one aisle but some do, as such an SOP has to be made that applies to all aircraft even the ones where it doesn't necessarily serve its originally intended reason