I think she is a throw away vp.
She is to controversial to actually get anywhere. She got stomped in the primaries. She isn't supported by the new radical left base.
So they think biden will lose, they don't want to waste someone that might be useful in the future, so they give her the spot, she gets to be the first black woman vp, she can try to raise her status as much as she can, if by some anti miracle biden wins he will most likely retire or something so she as vp might end up as president, and if biden gets stomped she was kinda a liability anyway so no great loss to the party.
She has nothing to really lose and potentially might end up as president.