Most people assume a plane thunders off the runway at full power every time, but that is rarely what actually happens. Commercial airliners never use full power on takeoff.
The takeoff thrust is calculated knowing the weight of the aircraft and fuel, plus an estimation of the weight of the passengers and the measured weight of their checked-in luggage, and also takes into account the temperature, air pressure and altitude of the airport.
The engines are the single most expensive part of a commercial airliner, many millions of dollars for each. Eighty percent of a jet engine's wear occurs in the takeoff phase of flight. Engine maintenance is a significant cost to airlines, so power settings are carefully designed to reduce engine wear. By not using full power, engine life is extended which results in lower operating costs.
Once the aircraft is off the ground and the gear can be safely retracted, the pilot can reduce the takeoff power to a more efficient climb speed. With the gear up and flaps retracted, the aircraft is in a clean configuration and can climb and cruise at a much lower power setting than takeoff. When the aircraft reaches cruise altitude, the power is set for cruise, where power can be reduced further to conserve fuel.
There is also a 250 knot limit below 10,000 feet, which means the pilot has no reason to maintain high thrust once airborne in controlled airspace. Some airports also require the pilot to throttle back for noise abatement until the aircraft is over less densely populated areas or has reached a certain altitude.
The takeoff power setting is often time limited. Pilots can only use that setting for five or ten minutes, and that is done to limit wear on the engine. If needed the pilots can continue at the higher power setting in any emergency situation.