First, to help motivate the Democrat base before the election to contribute lots of money and to vote early and often.
Second, to move the bar, so that any Republican wins can be reported after the election as far less than expected.
In 1994, the national lamestream media was caught unprepared for the Republican tidal wave that swept state and federal elections at every level. Remember all of the petulant "the voters are having a temper tantrum" reports?
This year, the media is already preparing their "the expected Republican wave failed to materialize" reports that they will broadcast regardless of how many races the Republicans win.
I agree with what you said. I would only add that if there isn’t a large wave, even if the Pubbies win the Senate by 6 seats,the LSM, Dims and Obummer will say that the Pubbies don’t have a mandate because there wasn’t a hugh and series wave; and, it was only a few of the red-state clingers and Bible-thumpers and the rest of the country loves Obummer and wants his agenda to move ahead. So, he and the Dims are going to continue to work for the American people and the poor undocumented children.