I may be remembering it wrong, but I think world was one of the few accepting people without a referral during the reddit migration. That's why I use .world. I also don't really understand federation besides it means I can see other instances, but I only barely know what that means. There were a lot of "what is the fediverse" posts when I first joined, and they were helpful, but maybe too simplified?

I also remember thinking .world and .ml were the defaults when I started.