When an aircraft brakes, kinetic energy of movement is converted into heat energy in the brakes. Taxiing increases the brake temperature. Landings are a high energy event, but the brakes had an opportunity to cool during the flight. Taking the runway for take off however, we must ensure that the brake temperatures aren't still too hot from taxi and the inbound landing an hour prior. The possibility of a high speed rejected takeoff will require the maximum energy consumption by the brakes of any operation. The hotter the brakes are to start with, the less margin for energy absorption is available.