Hillary knows that when Biden implodes, his votes need to go to someone. Hillary wants to be that someone. Otherwise, that support will go to Gabbard. The idea is for Hillary to jump in after the primaries on a white horse -- OK, on a white broom -- to save the day for the party. The wealthy donors have already said that they will bail if Sanders or Warren get the nomination. This is about money.
I don’t think many Biden backs would switch to Gabbard, who absolutely has a zero percent chance at the nomination.