A. afarensis is the species made famous by the "Lucy" skeleton discovered by Donald Johanson in 1974 in the Hadar region of Ethiopia. Previously, researchers had fossilized remains of the spine, hips, and knees of A. afarensis and developed a theory about how the species moved based on those fossilized remains. Researchers believed that A. afarensis was bipedial (walked upright on two feet, like humans).
Until recently the theory that A. afarensis was bipedal lacked certainty because while researchers were confident that the spine, hips, and knees supported the bipedal theory, A. afarensis foot bones had never been found. Without the foot bones, the researchers could not confirm just how the species walked, or how much it relied on walking.
In addition to supporting the bipedal theory, the foot bone and the fact that it is arched shows that A. afarensis would have been able to move quickly and easily on two feet. The arches in feet serve to help the foot move the body forward, and act as a "shock absorber" each time the feet pound the ground. The fact that A. afarensis had arched feet makes it even more similar to the anatomy of modern humans showing an important link in the evolution of upright walking.
I think this is such a great example of how science changes and revises itself because it showcases how very specific details and pieces of information can shape entire theories. In this case, finding the fossilized metatarsal bone confirmed a theory and also shed more light on how the species would have walked, providing additional information. It is a good example of science changing - and getting better.