In a Class III cavity preparation, the axial wall forms the base because it represents the surface of the tooth parallel to the long axis, providing a stable and supportive foundation for the restoration. The axial wall is critical in maintaining the structural integrity of the tooth and ensures adequate thickness of restorative material for strength. Unlike the pulpal or gingival walls, which serve as cavity boundaries, the axial wall directly supports the restorative base. Proper formation of the axial wall prevents overextension and protects the pulp from excessive removal.

Reference: Sturdevant's Art and Science of Operative Dentistry, James B. Summit, et al., 7th Edition.