Beveling enamel margins during composite resin cavity preparation primarily serves to increase the enamel surface area available for bonding. This enlarged surface enhances the micromechanical retention of the resin by exposing enamel rods at a favorable angle for acid etching and resin infiltration. As a result, the bond strength is significantly improved, leading to better retention and longevity of the restoration. Additionally, beveling creates a smoother transition between tooth and restoration, improving esthetics. However, the main clinical benefit remains the enhancement of the bonding surface area, crucial for the success of adhesive restorative procedures.
Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.