At the Gate when you board the dcs shows what is on the boarding card including seat number, the name and check in sequence.
At the end it also shows who has not boarded. If that passenger has any checked baggage or not.
If the passenger does not have any checked bags it easier to off load him. If he has and the aircraft is containerised then it's easier to locate his bags and off load it as the load control system tells which container it's loaded and the container position loaded on the aircraft. It's non containerised more difficult.
Furthermore the staff ( mostly load controller in most countries) has to sign off for passenger baggage reconciliation. I have seen one sacked because he off loaded a passenger and the bag came off but a loader put it back on the aircraft thinking it was a late arrival bag. The controller signed off and sent the aircraft. They discovered it after take off and the aircraft had to turn around and land and take the bag off. The controller lost his job.