I expect that each rally collects names, addresses, email addresses..... so Team Trump can motivate attendees to vote and recruit others to vote.
They get your email info to send the free tickets but that is the only information. In 2016 for theVirginia rally people came from as far as PA, MD,WVA,DC and OH. It was a late rally on a Sunday night also. It was an anti establishment surge and anti political correctness movement. The press/ media caused the Trump movement and that makes it all the sweeter.