Politician gets a poll-bump from the nomination usually, Democrats are hoping to shorten the campaign season (fewer chances for Biden to fuck up) and ride the september poll bump he gets into november.
I would be surprised if he actually shows up for the debates.
If you talk to liberals you'll find they see nothing wrong with a "multipolar executive". The constitution and the government are worth dogshit to them, it's all about 'winning' and by that I mean some fag with a barista job (or used to anyway, since starfucks fired all their baristas lol) feels like he's in charge by proxy when Democrats are in the white house.