There are three practical reasons Trump does this:

1. Deflection: Trump doesn’t have an affirmative platform. As a populist strongman, Trump’s platform is situational and entirely based on what his supporters want to hear in any given moment. If health care is in the news, Trump will say his plan is coming in two weeks (it won’t ever come). If immigration is in the news, Trump will say he will build a wall and get Mexico to pay for it (he won’t).
2. Deflection: Manipulating the media works. Trump knows that the more ludicrous things he says about Kamala, even if the media then starts to talk about how he’s wrong or fact-check him, the focus is still on the thing he said rather than Kamala’s platform. It’s subtle, but it really does focus the media effectively on whatever he says, and use his frame of that issue as the media’s frame.
3. Filling the echo chambers and other spaces. We’re in our own echo chambers like never before. Trump says these things so that the people in the right-wing echo chambers have a plausible response to Kamala’s policies, or even just need filler for their broadcast/websites/Facebook groups. Ultimately there is only so much media people can consume every day. If Trump has filled all relevant supporter spaces with his own opinions & framing, there is no time or energy left to explore other opinions and framing.