The Jewish religious scholars believe that the prophecies of the rebuilding of the Temple and the division of the land cannot be fulfilled unless someone is able to identify every Jews tribal linage.
They interpreted Malachi's prophecy that Elijah "will sit as a refiner and a purifier" to indicate that Elijah will return from heaven to supernaturally determine the tribal identities and linage of all the Jews worldwide.
They also believe he will identify who is qualified to be priests or Levites in the new Temple worship.