The behaviour of airplanes is not always the same; and weight has a lot to do with these changes. A heavier aircraft will need more thrust force and a longer runway for take-off. It will also have a higher stall speed and will also use more of the runway for landing.
Due to all of these, prior to each flight, pilots have to fill in load and balance graphs. In them, the weight of the plane has to be indicated, its on board fuel, the weight of the passengers and load, as well as indicting the temperature and altitude of the airport, so that the landing and take-off distances are obtained.
To conclude, it is of great importance for pilots to know all the weights the aircraft carries so they can optimise routes and altitudes and reduce flying time in fuel.