A lot of people polled (like 28% in one case) say they want to know more about Harris. This is not true for Trump. So, as long as Harris represents herself well in the debate, she has the opportunity to win lots of votes.
Yeah, that's how I feel about this "debate". Trump is going to do trump things, and this isn't going to change any cultist minds. But this could for her allow her to very publicly in a non-DNC setting allow her to (hopefully) hold her own and make herself known. I think the biggest thing (and why she absolutely should do it assuming she can) is prove she is mentally light years ahead of Biden, and de facto trump. If you replaced a bumbling Biden and the last thing many people have in their mind is that debate, you have to replace not just Biden as the candidate, but Biden's last memory to the electorate with a new memory of her. That is what she has to do. Prove she's coherent, able to handle zingers and send them back, and have it not devolve into who's golf game is better.
I think her campaign so far has been pretty good at keeping trump off his normal game, and if she can succeed in that and be again, coherent and mentally capable as well as clear, she will bring enough fence sitters over, especially if trump comes off as exceedingly boorish, rambling bully he really is.