The walls of an inlay cavity are prepared flared occlusally to provide adequate resistance and retention form. This flare allows for the proper seating of the inlay restoration and prevents its dislodgement under occlusal forces. Additionally, the occlusal flare facilitates easier removal of carious dentin and better adaptation of the inlay. Parallel or convergent walls may cause improper seating or lack of retention, while a tapered preparation without flare compromises resistance. Hence, occlusal flaring is essential for the long-term success of the inlay.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.