Besides what max_p explained, my reasoning was/is this:
I am a Reddit refugee. I did not want to have one corporation “in charge” of my account anymore on a platform. That is why I like the idea of the fediverse. Multiple instances make it more “resilient” towards “strange” decisions or decisions that don’t align with my ideas.
However, on one instance, the admin is the sole responsible (who does not even have to explain the decisions to a board or something). So making sure you are active on multiple instances counters their “power”.
I even went so far as to running my own instance. Because then I can be the crazy dictator if I want to.