The only direct evidence that Trump knew about the scheme to falsify the business records is Cohen's testimony. So if they can convince jurors that his testimony can't be trusted that might given them reasonable doubt as to Trump's knowledge of the scheme. And they definitely showed that he has lied about a lot of things in the past. Not saying it's enough to doubt his testimony (not like we weren't expecting it), or to conclude there isn't enough other circumstantial evidence that Trump knew they were falsified, but it's the biggest glove they've laid on the prosecution's case so far.
The thing is, even if Cohen is slimey and has lied about some things, the point at the end of the day is - you don't need to believe Cohen 100% in everything he says, you just have to believe Cohen when he talks about the Trump records part of it. And given the other circumstantial evidence that supports Cohens claims, that at least backs up Cohens claims about that situation, outside of whether Cohen is telling the truth about other things.