Now these are the people of the province who came up from the captivity of the exiles, whom Nebuchadnezzar king of Babylon had taken captive to Babylon (they returned to Jerusalem and Judah, each to their own town…Ezra 2:1

            Family records were important to Jews because of the priesthood and the lineage of the kings from Judah.  You had to know who you were in order to serve.  When certain people’s family line could not be traced back to Aaron (or it was unclear), they were excluded from service.  Restarting the temple was the whole reason they returned to Jerusalem so the priests needed to be certified.  Since they were also reestablishing a lost city, the records helped to know who was who.