The inside politics reason that Harris exists in the white house, is that the Hillary faction of the DNC (who hates obama, almost as much as they hate bernie), were positioning Harris as their replacement for Clinton. After Harris face-planted repeatedly during the primary and it was obvious she was never going to be elected, she joined the centrist voltron along with Pete Buttigieg, Bloomberg, Amy Klobachar, and Elizabeth Warren who all dropped out and endorsed Biden right before South Carolina in order to stop Bernie. Pete got to be secretary of transportation, Harris got VP.