Leslie A White, a cultural anthropologist wrote a fascinating book: 'The Evolution of Culture - The Development of Civilisation to the Fall of Rome' (1959), (sadly out of print), where he explains the cultural purpose of these people. Here is an excerpt:
"...Let us turn first to the qualifications of a shaman.
Traffic with the supernatural world is by its very nature mysterious. Extraordinary abilities are therefore required or are at least an asset. Epileptic fits, trances, hallucination, and hysteria are mysterious experiences, and hence eminently suited to communication with the supernatural world. Dreams during trances are interpereted as visits to the land of the spirits; fits and hysteria are states produced when a spirit seizes and possess the body the shaman. This conception is preserved in our word "epilepsy" (Gr. epi, upon; lambano, seize). Hence neurotic, abnormal, and unstable individuals are better qualified for the profession of shamanism than are normal and stable persons. As Tylor long ago observed: '...In all quarters of the world the oracle-priests and diviners by familiar spirits seem really diseased in body and mind...' More recently, Lowie has commented upon the abnormal character of the shaman: 'From Africa and Oceania, from Siberia and Tierra del Fuego we thus have evidence that shamans are either abnormal or at least temporarily capable of passing into abnormal mental states.' The advantage of abnormality in trafficking with the supernatural world is further indicated by the widespread use of drugs, liquor, fasting, self-torture, and solitude to produce temporary pathologic states as a means lor a condition of supernatural experience. Thus we see that some individuals are better qualified to become shamans than others."