Why do airlines require passengers to return to their allocated seats for landing? Airlines which allocate seats often make announcements prior to landing to request passengers return to their allocated seats. What is the motivation for this requirement, especially since I have never seen the requirement rigorously enforced by the crew (by requesting sight of the boarding pass).



The requirement is not simply a polite way of requesting standing passengers return to their seats – for two reasons:


Crews are typically explicit in their language when giving directions, such as requiring standing passengers be seated. It is clear from the phraseology and emphasis given in the announcements that returning to the allocated seat is a critical component of the request.
On a recent long-haul flight, a couple seated in my row were offered to relocate to vacant seats in an exit row for the duration of the flight, provided they returned to the allocated seats for landing. (They were not, at the time, standing.) Furthermore, in the event, only one member of the couple returned; the other remained in the non-allocated seats (an example of non-enforcement).




Some possible motivating factors I have considered:


Weight distribution – ensuring the flight envelope of the aircraft is not exceeded during critical phases of flight. However, I have encountered this situation on wide-body jets on long-haul flights (e.g. A346, A388), where a handful of passengers, of nominal mass, being situated elsewhere in the plane does not seem cause for concern.
Collection of personal effects – if passengers relocate after takeoff, they may leave personal belongings at their original seat, unwittingly or not.   A requirement to return to the original seat may:


trigger passengers to locate &amp; take with them items left/forgotten at their original seat, reducing the cost of the airline returning these to passengers later.
improve the efficiency of disembarking by avoiding passengers who need to move against the flow to collect belongings elsewhere in the aircraft. 

However, I have encountered this after long-haul flights with tens of hours of turnaround time on the ground, so eliminating a few minutes from the disembarking time of some passengers does not seem a high priority (and, in any event, the effect of reducing the overall exit time would be in the noise and difficult to quantify).

Limitation of privilege (hence, revenue protection) – passengers in a higher cabin tier may benefit from certain privileges upon landing, such as the possibility to disembark the aircraft more quickly. However, crew are normally hot on ensuring passengers do not self-upgrade to a higher cabin, so this would not seem to apply.
Emergency preparedness – for example, ensuring there is a sufficient cabin crew to passenger ratio in all areas of the aircraft to meet emergency / evacuation requirements. Such ratios may be upset in the event passengers relocate far from their original seat. (If so, why is this not an issue when in the cruise?)
Passenger identification – air crash investigators often use seating manifests to identify remains of deceased passengers, so this requirement could be to ease any future investigation in the event of an incident on landing. (Similarly, what about the rare likelihood of an emergency which rapidly develops while in the cruise, without opportunity for passengers to relocate?)


None of these reasons seem to entirely justify the requirement for this routine request or its existence across all manner of aircraft types.



My questions


What benefit do airlines or the wider industry gain from this requirement?
Why is it not rigorously enforced by crews?
How do airlines which do not allocate seats to passengers satisfy the same requirement when there is no published seating manifest in advance of passengers entering the aircraft, or why are they exempt from such requirements?
Its a matter of legal requirement (under the FAA, and I presume the EASA has a similar reg). Strictly speaking its really more important that the passenger returns to a seat. Generally you are asked to return to your seat as it can be assured its empty and available for you. Under FAA regulations you are required to be wearing a seatbelt for landing and thus must be in a seat 


  14 CFR 121.311 - Seats, safety belts, and shoulder harnesses.
  
  (b) Except as provided in this paragraph, each person on board an
  airplane operated under this part shall occupy an approved seat or
  berth with a separate safety belt properly secured about him or her
  during movement on the surface, takeoff, and landing. A safety belt
  provided for the occupant of a seat may not be used by more than one
  person who has reached his or her second birthday.


Weight and balance is important but not on a single passenger level i.e. on an airliner of decent size if a single passenger moved around it would be ok. If half the plane was empty and the passengers rearranged them selves there might be an issue.  


  How do airlines which do not allocate seats to passengers satisfy the
  same requirement when there is no published seating manifest in
  advance of passengers entering the aircraft, or why are they exempt
  from such requirements?


They are not exempt from the requirements but there are enough seats for everyone on the plane and as noted above its really important that you are belted in so taking an available seat will do in this case. On these flights generally speaking, you take a seat prior to takeoff and that somewhat becomes your seat for the flight. In my experience there is little if any mid-flight seat swapping.    


  Furthermore, in the event, only one member of the couple returned; the
  other remained in the non-allocated seats (an example of
  non-enforcement).


As noted above the policy is to be belted in, so the regulation was indeed enforced. The other member of the couple may have remained in the seat if they were the only person capable of preforming the exit row duties.