Maybe a convenience, but not necessarily better....The physics involved ?
A sling bag is usually carried from shoulder to side, varying left or right (shoulder to side) which would cause one to not be centered (balanced). This loss of balance can cause aching, sore muscles, and fatigue.
A backpack however is most usually worn on the center of the back, i.e., better balance. Less physical stress on body. Also, if the back pack is worn high on the back, the weight of it tends to cause you to stand up straighter; preventing stress on the back and spine. The lower the backpack is worn, the more often you'll have lower back pain, and aches.
