In case you're not being facetious, adoption begets adoption.

People use it because some people they like to follow aren't on Twitter

People use it to communicate with people who aren't on [insert competing messaging platform] because it's easier than convincing every friend to move to your preferred one.

People use it to track events because it has wider adoption than Eventbrite/etc. (and that platform choice lies largely with the organiser rather than attendees)

etc.

Because we have social lives? It's where events are organized, photos are posted, current issues discussed, and friends stay in touch. It's really not that hard to understand.