Democrats conceded the legislation could not pass the current Senate, where it would need 60 votes, and has no prospects in the Republican-controlled House. But they said the debate would focus attention on ethics issues on the Supreme Court and could build momentum for future action by Congress.
Republicans oppose it because that what they do. Chuck could say that it was sunny outside and Lindsay would make a show of bring an umbrella just out of spite.
In an environment like this, when very little can actually get done, sometimes activity like this is done to set the stage for the next thing. Democrats will campaign on this to sell voters on the idea that the Supreme Court is out of touch and unaccountable, and Republicans are standing in the way of changes. And if the Democrats win majorities in both houses in this election, I fully expect them to nuke the filibuster to pack the court.
My preferred method would be to slam in expansion to 13 on Day 1, effective in a years' time. And then after signing it, Biden can go to Republicans and say "You have a choice: you can work with us to reform the court via amendment: institute ethics requirements, term limits, privledged status for appointments in the Senate, and efforts to make the Court less of a political football and more accountable. Or, you can leave things as they are, I will appoint 4 young judges to lifetime appointments and you can gamble on having both the Presidency and Senate control to appoint any more."
Because you know that if the Presidency and Senate are controlled by different parties from now on, the Senate Leader will invoke the "McConnell Rule" to ignore the appointment entirely. In fact, this can be used as a justification to go to 13, because Democrats can argue that the Court will often have vacancies, because the Senate Majority leader has a permanent veto on filling the seat.