Friction between the foot and the ground is a basic physical necessity for terrestrial locomotion, enabling the generation of forward propulsive forces without slipping.
The claim that friction is the primary force enabling human walking is a fundamental physical and biomechanical truism. Walking relies on static friction to push off the ground and advance the center of mass; without it, foot slippage prevents forward progression (as explored in broader contexts by papers discussing slip and terrain friction like [4] and [8]). Because this is an established physical observation of everyday bipedal mechanics, no specific citation is needed.