It's always the spectacle. Trump doesn't visit blue states to flip voters or fire up a base that's already locked in — he goes for the cameras and the storyline.
California is enemy territory for Trump, which makes it perfect TV. He gets conflict, headlines, and a viral spectacle without needing to win the state.
It's entirely about the media spectacle. Going to a deep-blue state guarantees wall-to-wall coverage and forces the news cycle to revolve around him.
Trump visits California for the cameras and chaos. It's all about owning the libs and dominating headlines in enemy territory.