Proposed medical purposes include treating head wounds, skull fractures, epilepsy and hydrocephalus; proposed spiritual purposes include releasing evil spirits believed to cause illness and producing bone-fragment talismans. Neither explanation can be confirmed or ruled out from skeletal evidence alone, and the two purposes are not mutually exclusive across the many independent cultures (Neolithic Europe, ancient China, pre-Columbian Peru and Mesoamerica, East Africa) that practiced trepanation. A widely repeated claim that it treated headaches has no credible supporting evidence and traces to a 1913 misreading by the physician William Osler of the anthropologist Paul Broca's earlier writing.
What would resolve this Direct textual or iconographic evidence from a trepanning culture stating its own purpose, or a rigorous cross-cultural study correlating trephination sites with skeletal trauma versus other markers, at a scale sufficient to separate medical from ritual practice.
OpenArchaeology and Medical AnthropologyWikipedia: Trepanning