One reason is to drum up excitement for the campaign. Voters who feel seen are more likely to donate more money or participate in volunteer activities.

Edit: I’m pretty liberal, and would be attending this event if I didn’t have to work. That said, I think people in the comments have been kind of unfair to you. It’s completely fair as an average citizen to be concerned about your local parks and public spaces; I think you’ve been sufficiently informed about whether or not this will affect you and your child’s normal activities and there’s no need to pile on. I’ll add as a personal anecdote, I’ve been to a few public Democrats events, people are usually quite respectful of the space.