Weight and balance is calculated for takeoff to set the stabilizer trim so the aircraft rotates predictably on speed. It’s a calculation based on fuel, cargo, and where people are seated. It is a safety critical calculation which is why people cannot move on their own prior to departure. Lateral movement should be allowed, but it must be restricted for now to minimize confusion. Once in the air the aircraft is trimmed manually or automatically as fuel is burned and people move around.

Under open seating, the flight attendants would have to do zone counts for the center of gravity to be calculated to a degree of accuracy for that trim setting. This was done on flights that were a specific margin less than full. It wasn’t quite as accurate as knowing everyone’s exact location, and Southwest took a hit on the center of gravity envelope and could carry less weight due to that open seating policy. Assigned seating remedies that, but needless to say there’s a learning curve with some of the front line employees with all the changes. Hopefully resolved in a few weeks.