Aircraft weight is important not just for calculating things like how much runway, thrust, and fuel an aircraft will need, but also for other things like calculating how much they disturb the air by creating vortices. These vortices can be dangerous for other aircraft flying in their wake. Commercial aircraft are divided into weight categories by the ICAO to account for the Wake Vortex Turbulence they generate. The FAA also has categories for aircraft based on weight that mirror those of the ICAO, except for smaller aircraft.