What benefit would an improved muscle tone from strengthening the quadriceps femoris muscles provide?

The answer to this question would be: better-stabilized knee joint

The quadriceps femoris muscle is located in the anterior part of the femur. This group of muscle is used for extension. It has no role in flexion, abduction or adduction. In general, training a muscle will stabilize the nearby joint. In this case, the quadriceps will stabilize the knee joint.