Retain some credibility? He’s just standing there letting her campaign. This isn’t a town hall, one of her staffers asks a question and she rambles for five minutes her tired old slogans. She avoids even when he tries to get a specific answer and he never pushes harder. This is a campaign event, Trump was right to skip this.
Trump is winning. So he is playing it "safe".
Harris is having to do more interviews, town halls, etc because we are not sold yet on who she is.