Ruthie Paine got him this menial labor job after Oswald had been fired from his last one. She felt sorry for Marina Oswald and their new baby and tried to help out by pulling strings with an old friend at the TSBD to get Lee some work.
If you don't believe that, please show your conspiracy chart that shows how this kindly Quaker housewife fits in with the shadowy CIA/KGB/Mafia/LBJ conspiracy puzzle to eliminate Kennedy.
By the way, Ruthie Paine is still very much alive.
Maybe you should ask her how and why she did it. Most conspiracists try to ignore the Paine family entirely because their very presence is a weak link in their fantastic conspiracy theories. To most conspiracists, Oswald just materialized in the 6th floor window of the TSBD, rifle in hand waiting for Kennedy's motorcade to pass by.