When done with attention, walking can improve the posture. Standing up with your back straight can make you look younger immediately. The posture influences the appearance. An erect posture is often perceived as being young and active.
Influencing the appearance is just a by-product. Postures have a great effect on holistic health.
When one walks at a fast pace, the muscles that are used to sit up erect and carry oneself have to put in extra efforts to keep up. Though simple walking can strengthen these muscles, it is even better to focus on the posture while walking.