1. Bernie is doing it because he wants Bernie or Busters to vote for Biden, because he thinks it's imperative that Trump is removed from office come November.
2. Joe Biden's platform is slightly progressive, but no where near the progressive platform of FDR No medicare for all, no legal marijuana, no free college, no defunding the police etc. There's no doubt there are some progressive aspects to it, but they'll be watered down a million times over once he's in office.
3. Unfortunately, the way I see it, the influence from the left will be trumped by the influence from the right and the corporations and lobbyists, given Biden's establishment. If he becomes president and genuinely moves to the left, I won't be complaining (You probably will). But it's probably not gonna happen.
3. Communists are endorsing Joe Biden not because he's a communist (lmao), but because he's a lesser evil. They hate his guts, but they hate Trump's guts more. Also, if someone endorses you, it doesn't mean that person shares your ideological beliefs. David Duke, a rabid neo-nazi endorsed Donald Trump. Is Trump a nazi? Asking for a friend.