What is the primary purpose of using left and right brakes during landing?

The primary purpose of using left and right brakes during landing is to control the aircraft's yaw. This maneuver is crucial because it helps maintain directional control of the aircraft on the ground, especially when the aircraft is transitioning from flying to rolling on the runway.

When a pilot applies brakes selectively—using the left brake to slow down the left wheel and the right brake to slow the right wheel—they can create a differential braking effect. This differential application helps counteract any yawing moments that might be caused by factors such as crosswinds, asymmetric thrust, or even the natural tendency of the aircraft to drift. Maintaining a straight path during landing is essential for ensuring safety and stability, especially when nearing the end of the runway.

The other options do not pertain directly to the primary function of the brakes during landing. For instance, improving fuel efficiency is unrelated to the braking process, as fuel consumption mainly concerns engine power settings. Initiating a controlled descent is typically achieved through control surfaces and throttle management, rather than through brake use. Lastly, reducing engine power involves throttle manipulation rather than the application of brakes, which serve more to manage speed and directional control post-touchdown.