Trump does all these off-year rallies for two reasons:
1. He enjoys them. I don’t think I see pictures of him smiling while he’s doing his job, but there’s always a big goofy grin when he’s onstage for his true believers. He can say anything there, and they love him for it.
2. It was reported back in 2015 that Trump has had a longtime Hitler fixation - not necessarily that he admires the man’s deeds, though there’s probably a good analysis to be done on that basis but that he’s done a fair amount of reading and study of him. I strongly suspect that he thinks this sort of rally is what a strong leader does.
And the ideas reinforce each other. “Look at my rallies, aren’t I great?”
They are ugly, and frankly unamerican, but I don’t worry about them directly because they’re such a circle-jerk. I worry that there are places like Fox that cover them uncritically. I worry about that a lot.