What I mean primarily is, that with all three branches they could (I think) quite easily force through changes to the supreme court and destroy Trump's main (only?) legacy. Without that they couldn't.
I suppose that fixing the system so that they don't come so close to toppling into fascism ought to be a priority. And these would involve big, controversial changes of the kind that they will never be able to negotiate some kind of compromise with the GOP, they will only be possible if forced through. Re-jigging (or getting rid of - though that won't happen) the electoral college to be more in line with the popular vote say.
Also it will mean that you won't have a senate constantly refusing to sign off the budget and just stopping government working as has happened so often since Obama.
Just to enlarge on that. Dems have won a majority of the vote in 7 of the last 8 elections but have lost several of the elections and have thus appointed virtually none of the supreme court judges - though Trump appointed three of the nine in his single term. The senate votes are also counted really unfairly, in the last mid-term the Dems increased their vote share and decreased their representation.
I think if at all possible the Dems need to do something to challenge this - I think there is a justification to bringing the electoral college votes more in line with state populations and thus the popular vote. Why should California votes count less than others? But the GOP will NEVER countenance that - it has to be smashed through them.
Same with the boundaries drawn unfairly for the Senate, the GOP will never agree so it can only be improved by a dem trifecta I suspect.
Also if they have they have the trifecta I think they would be totally within their rights to throw Barrett off the court (as she was imposed against the rules that McConnell invented last time) and impeach Kavanaugh. Alternatively, apparently the reason there are nine judges is cos there were nine circuit courts and there are now thirteen - so there would be plenty of justification to add four liberal justices to the court. There would be howls of rage but - as with the Trump regime refusing subpoenas, not allowing evidence at the impeachment, forcing through Barret etc - they could just ignore it and tell them to fuck off. And with a lot more justification.
So for me it's important to make these changes so there can be a level playing field - I'm not talking about specific policies for Biden, but I'm saying that unless this kind of thing can be done then there is no chance of a level playing field and there is no chance of policy reflecting the make up of the country for the next fifty, hundred years....