Gotta go where the people are. If they truely believe there are republicans that will vote for them because he is that alternative to the Trump they are tired of (an admittedly small amount), this is as good of a place to go to speak to them. The country right now is biased enough that the margins of swing are super small and if this can help attract those people it makes sense for a campaign like Biden who has maintained that there’s a subset of republicans that are non-MAGA and just want to get back to “normal”.