People from different instances are not able to login on your instance. For example you might see me in the people table, but local should be false. People registered on your instance shows up on local_users and on people with local set to true.
There have been mass registrations going on recently. Imagine my surprise when my 3 people instance had 460 users but no one online today. Took my instance down to investigate and deleted all local users in person table except the known users. Not the cleanest way to do it but I don’t think I broke anything :D
To have some control over my data and for the fun of it. With money? Selfhosting to me is a hobby and it has also taught me a lot, some of which has been helpful on my career.