The main function of pins in an amalgam restoration is to increase the resistance and retention of the restorative material. Pins provide additional mechanical retention by anchoring the amalgam into the dentin, especially in cases with insufficient tooth structure to support the restoration. This helps to prevent dislodgement and improves the longevity of the restoration under occlusal forces. Pins do not affect esthetics, microleakage, or finishing but are primarily used to enhance the stability of the amalgam filling.

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