It reminds everyone that Harris lied about working at McDonalds. And while it seems to be of little significance, it’s such an obvious lie. People don’t like being lied to in such an open way.
The big thing Trump said was that he worked there 15 minutes more than Kamala which forces the media to remind the public of the lie.
Even more, it illustrates how personable and likeable and normal and funny and smart Trump is.