Aircraft with retractable landing gear have electrical pressure switches -- often called weight-on-wheels switches -- to prevent retracting the landing gear while on the ground. When lifting an aircraft on jacks, the switch opens, allowing normal operation of the landing gear as if the aircraft is in flight, which has the potential of injuring the personnel lifting the aircraft. Disconnecting the battery renders the landing gear system inoperative to provide a safe work environment.
Aircraft engines, much like automobile engines, are started via an electric starter motor powered by the battery. The electric starter motor has enough power to turn the engine and propeller with enough speed and force to cause serious injury to anyone entering the propeller's arc. Disconnecting the battery prevents the starter motor from engaging.
Avionics such as communication and navigation radios, flight management systems and instruments are electrically powered, expensive, sensitive pieces of equipment. Disconnecting the battery prevents damage to the avionics and electrocution during maintenance.
