It's not about those voting for him, they're a lost cause.

It's the fact that if you don't vote for Biden, based on the current options and possible outcomes available, you are saying that you're okay with Trump beating him.

US elections basically come down to turnout of Democrat voters.

The GOP has baked in voting as a civic duty and party loyalty as core values, so GOP votes mostly change one casket at a time.
Dem voters are a lot more flighty, and a lot more willing to refuse to vote or vote third party if the Dem candidate isn't everything they want. There are more Dem voters than GOP voters, but fewer of them actually vote in any given election.

Which means that GOP turnout is pretty stable and Dem turnout is all over the place.

A big part of why Trump lost in 2020 is that Dem messaging was all about inspiring terror in their own voters that Trump might win again, so get your asses out and vote or else! Same as now, and it might hold as a tactic until there's a GOP candidate they can't successfully work the fear angle on, and then the GOP will win and if the US isn't destroyed in the ensuing four years Dems will have a hard time winning again until they find a new tactic.