The primary function of rests in a removable partial denture is to transfer occlusal forces along the long axis of the abutment teeth. This helps to distribute occlusal loads evenly, minimizing trauma to the soft tissues and reducing stress on the residual ridge. Rests provide vertical support, preventing the denture from sinking into the mucosa during function. By directing forces toward the abutment teeth, rests protect the underlying tissues and maintain the stability and longevity of both the prosthesis and the remaining dentition.

Reference: Stewart's Clinical Removable Partial Prosthodontics, Stewart M., 4th Edition.