Behind the scenes of every flight the pilots are preparing the flight by conducting a weight & balance computation. The importance of  weight & balance is to ensure that the aircraft is not over the max weight certified by the manufacture and to make sure the weight is distributed in a way that it dont vastly effect the flying characteristics of the airplane. In other words, making sure the plane don't crash. Engineers build airplanes with a little leeway or fudge factor, but pilots know not to ever exceed that fudge factor because you never know if something else may come up expectedly. Which leads me to discuss the second issue with weight & balance. Besides making sure the aircraft is not over weight, we also want to make sure the aircraft's center of gravity intersects right through the center of the airplane or wing area. This ensures we are properly balance. If the weight moves too much either forward of the fuselage or afterward of it then this has an effect on the center of gravity (CG). Lets say the baggage handlers placed a thousand pounds of weight in the cargo bin with out telling the flight crew. Will the airplane crash? Probably not! The airplane can still fly with a CG out of range. The problem is if something else happens to go wrong such as an engine failure on takeoff. Now the combination of these two things may make a huge difference in the safety flight. Every aircraft has to compute weight & balance including large jets. Sometimes on occasion a flight attendant may have several passengers move their seats due to weight & balance. Please be patience & comply because this is a safety logic behind everything.