Onias iii was one of the last high priest in the Temple in Jerusalem. He was deposed as Judeans and Greeks tried to Hellenize Jerusalem. He took refuge in a temple in Syria where he was murdered. His son fled Judea and took refuge in Egypt, where the reigning Ptolemies constantly laid plans to retake Judea from the Seleucids ruling Syria. 
As the rightful heir to the High Priesthood, Onias iv could be a potential asset for the Ptolemies in some future escapade, at least in controlling the Judeans themselves if they could wrest Judea from the Seleucids. Ptolemy gave Onias land to build on as well as land on which to earn his own upkeep. Onias set about to build a Temple to the One God of Judea. Let that sink in. The rightful heir in the line of Zadokite high priests stretching back to the Persian restoration of the Temple built a Temple to God outside of Jerusalem!