Exactly. It’s not about the attendees at the rally. It’s about the media coverage of the rally.
Going into the actual vote, a candidate wants to project enthusiasm and momentum. Most voters are not very smart or sophisticated. Many, many people function at an intellectual level barely above dumb herd animals. They want to join a winner. They want to be included in the flock. They don’t root for the Chiefs because of team dynamics or Reid’s coaching philosophy or whatever, they root for the Chiefs because the Chiefs are winners (and previously the Patriots, and before them the Cowboys, and before them the Packers, etc etc).
So the candidate needs to maintain a constant output of large, happy, cheering crowds. If that flags to any perceptible degree, it gives the perception of failing momentum. It projects an impending loss. And the voting herd will get skittish and run away.
This is why Trump doesn’t really give a shit that he’s bloviating to the same mob of traveling Deadheads. It’s not about them. It’s about having pictures and video where he holds forth and masses of people celebrate him. And it’s why he and his handlers are so obsessed with crowd size and making sure the camera angles obscure any gaps and maximize the impression of attendance. I mean, yes, for him, the higher priority is his own ego gratification, but he also knows this is a key aspect of his image management.
Harris has to respond in kind, but she understands the strategic campaign message is the primary driver.
And, ultimately, the rally is a campaign expense and it does occupy a block of the candidate’s schedule, but it’s not really that time-consuming, in the wider context. Harris gets up and gives her standard stump speech for, what, 30 or 45 minutes, depending on the local details (deadline for travel getaway, who else is on the stage, etc). The actual on-stage time is conducted largely on autopilot, with a handful of local customizations tossed in here and there (in this city we mention corn, in that city we mention steel). There’s no new memorization or rehearsal. At this point, it’s almost entirely pat.
And further consider that backstage, Harris is occupied glad-handing the big-money donors before she steps out for her speech. As she travels from town to town, the hour or maybe two where she has to be on camera will be outbalanced by the many additional hours where she’s making a private appearance at yet another wealthy person’s private house, soliciting donations and support.
So — rallies are just the public-facing justification for the travel, but they’re not necessarily the primary purpose.