Why do you have to get permission from the Captain to move to a vacant seat?

The aircraft's speed is calculated by the weight and balance of the aircraft, and many factors need to be considered for a successful take-off. Factors include the weight of passengers and where they are seated, the weight of the cargo, freight and fuel, the distance available on the runway et cetera. For example, if there are 100 or more kilograms of freight in the cargo, the balance of the aircraft will be better maintained if passengers are seated in the forward rows.