Because fewer votes for Biden emboldens the DNC less both their own eyes and in public opinion. No matter who wins, it's best if the election is a show of support for neoliberalism to the least extent possible.

Of course, its seeming more and more that Trump actually went too stupid for the American public, which didn't seem possible, so I think there's a good chance Biden will pull off a powerful performance based solely on people not wanting to die of coronavirus under Trump.

I think the biggest sign of hope as far as electoralism goes is the fact that Biden is being characterized by Trump as some sort of anarcho-communist, and he's characterizing himself as "almost as good as Bernie." Even though both of those are extremely far from the truth, if people think Biden is somehow left wing then a big win for him won't be seen as a rebuke of the actual left.