Running is whole body exercise, not just an action performed by the legs. A good endurance runner depends on upper body as well lower body strength.
For example; during the running stride muscles in the arms, abdomen, and lower back repeatedly contract to absorb the shock of landing and counteract rotational and side to side movement. Poorly conditioned calf muscles can cause flat-footed running and heel striking. Weak hip flexors may limit the height of knee lift.
We want to strengthen these areas to reduce injury and improve our running mechanics. If we do so, we become stronger and more efficient runners thus improving our overall performance.