You have thus seen the reason why ailines board their passengers as per normal when there is a delayed slot. There can be changes in air traffic flow rates creating more slots and other aircraft may have to cancel their departures thus making their previous slot available to others. This can all happen at only a few minutes notice. Aircraft with their pax on board can take advantage of this. Those that leave their pax in the terminal will find that it takes about 30 minutes to board and thus miss out on any short notice slot improvements.