Debates really only matter when the candidates weren’t really known to the general public / people who weren’t engaged with the primaries.
Not really an issue when one candidate is the president and the other candidate was the president and so people’s opinions are all but locked in.
It's definitely possible for someone to be complete, farcically unprepared for a debate (e.g Peterson vs. Zizek), and yeah, listening to two people having a stand up discussion for a hour definitely helps give you a sense of where they're at, especially if they're previously unknown to you.
Unfortunately, for those undecided voters and people who have been inundated with messages that Biden is old & feeble, this debate will serve as proof (though only one point in time) that it’s actually the case.