The importance of weight and balance control, from an aviation standpoint, leads to the safe and efficient operation of an aircraft.
During the design phase of an aircraft, the manufacturer tries to make the airframe, the structure of the aircraft, as light, yet as strong as possible. The greater the weight the aircraft, the stronger the aircraft structure has to be, meaning, an even heavier airplane. This is because an aircraft has not only have to deal with static loads on the ground, it also has to deal with dynamic loads (turbulence, lift generation from wings) while in flight.
Performance also decreases exponentially as weight increases. Not only are the stresses greater because the wings have to generate more lift to counteract the excess weight but also higher take-off and landing speeds and distances will be required. Range, endurance, maximum ceiling, rate and angle of climb and maneuverability will be decreased. It is due to all these factors that aircraft are specified with a maximum take-off weight (MTOW). This weight is the maximum at which the aircraft may be legally operated. It is however noteworthy that MTOW is only a design consideration. Additional operational considerations, such as a short runway and high temperatures and altitudes (which mean less dense air) may lower significantly an aircrafts MTOW.
The Center of Gravity is the point at which gravity is said to act on a body. The position of the Center of Gravity, with reference to the point where all the lift is generated has an effect on stability. The Center of Gravity is constantly changing. The effect an object has on moving the Center of Gravity is not only a function of its weight, but also the distance it possesses from the fulcrum. In general, the longer the distance, the greater the force. Fuel burn, different passenger weights, presence of cargo among many other things move the center of gravity. The manufacturer, during the testing phase of a new aircraft determines the range that the center of gravity can fall in. The accepted flight envelope is set between the forward limit and the aft limit.
If the Center of gravity goes too forward, the aircraft will have a nose down pitching tendency, efficiency decreases. This will reduce range, endurance and cruising speeds. If the Center of Gravity goes too far aft, the aircraft will then become longitudinally unstable. Therefore, the correct balance must be maintained.