"Always have a plan B."
The Airbus family airplanes generally incorporate two autopilots in their airplanes. AP1, which is associated with the left instruments, is normally used by the captain which sits on the left hand side, whereas AP2 is used by the copilot on the right. Depending on who is Pilot Flying, the corresponding AP will be used throughout the flight. On some occasions, such as during ILS approaches, both autopilots will be engaged to provide a fail-safe mechanism. If one of the autopilots fails, the other autopilot will immediately take over and maintain control. The main purpose of the autopilot is to relieve the pilot’s manual flying effort (imagine manually flying across the Atlantic Ocean). It controls both lateral and vertical trajectory of the airplane, as well as its speed via auto thrust. As such, the pilot will have more capacity to perform high level managerial/decision making roles. While being a useful tools for pilots, it follows the GIGO (Garbage in, garbage out) principles. If the pilot insert the wrong heading/alititude, the autopilot will just follow the instructions faithfully and blindly (albeit with various warnings built in).