Wear happens from below
When a rug sits directly on wood or tile, every footstep compresses the pile and presses the foundation against an unyielding surface. Grit trapped underneath acts as an abrasive between the two. The foundation wears through from the back while the face still looks acceptable, and the first visible sign is usually pile shedding or a section where the backing shows through. By then the damage is structural and repair is expensive.