So you board the aircraft and you are lead to your seat by the cabin crew. You stow your luggage and take your seat. Then right when the aircraft door is closed and armed, you noticed that there are quite a number of blank seats in the middle area, and you are at the back with other passengers, all cramped. You are thinking of moving forward where you have the whole row to yourself. Then just as you inform the cabin crew that you want to take the vacant seats, the crew informs you that you can only change seats when the aircraft is in the air and that they will have to inform the pilots first. Why? What's the difference between now and later?
There is actually a huge difference in which the safety of the aircraft will matter. The whole weight balance of the aircraft will be totally up-ended if many passengers decide to follow you. Take-offs and landings are the most crucial part in a flight, in fact, most crashes happen during these moments. Your aircraft's center of gravity is very important during take-off and landing, and pilots need to know the weight distribution. Before they take-off, everything is computed to the last detail so they know how much thrust to apply and so on.
Before take-off, the pilots inputs data into the computer system of the aircraft. This includes weather condition, wind speed, fuel load, and payload. Included here are the computation are the distribution of the passengers in the aircraft and the cargo arrangement and weight. So basically, the pilot will compute and input everything based on your original seating arrangement on the boarding passes. Hence, if passengers change seats without informaing the pilot and the weight distribution is not the same as what the pilot have inputed on the system, the whole airplane can crash during take-off!
A pilot shared that during one of his flights, four passengers seated themselves a few rows forward of their assigned seats. During take-off, particularly during the rotation stage, the pilot felt that the aircraft seemed very happy, unable to take-off right away. This was a critical situation because the runway was relatively short and it would be too dangerous to suddenly stop the aircraft, it would need to take-off not matter what. Well good thing it still did.
So next time, please avoid treating an aircraft like a bus on the road. Buses do not fly, but airplanes do! Before swapping or being reseated somewhere else, please always ask permission first from your cabin crew. Do not just stand-up and seat somewhere else. It's for everyone's safety on board.