Updated: Oct 29, 2020
Airlines assign seats for a few reasons.
It gives the airline a manifest, who is in what seat.
It is vitally important to know who is in a certain seat for safety and security reasons.
2. Weight and Balance,
The Balance of an aircraft is extremely important to the take-off and landing. The whereabouts of the passengers are crucial for the pilots to input correct numbers If passengers move after the final numbers it could cause a big problem for the performance of the aircraft.
3. Surcharges are paid by customers
Airlines Charge for premium seats. When you purchased your ticket you were asked if you wanted to pay for a certain seat. If you declined you get what is left over after all seats have been purchased (usually at the gate and usually in the middle seat.). You cannot get into a seat after the doors are closed for the previously listed reasons AND those other around you PAID to be in those seats.
If you want to change your seat, do it prior to boarding. As a gate agent, I have some discretion of not charging, if you are traveling with a child, elderly, and in need of assistance. But other than that a premium seat will cost money. Airline agents do not make the rules but have to adhere to them. The airline company we work for is in business to make money and that is how they do it. We really do not like to but we are paid by our company and have to abide by it.