it basically comes down to personal preference tbh, each instance have slightly different rule sets and internal communities, lemmy.ml / lemmy.world are simply some of the 'main' instances, They're higher on the list on the instance search and thus generally gather a larger crowd because people see the higher user count and just go with that.
and if you don't like any of the public instances you can simply roll your own and use it as an internal friends chat that can also browse the external fediverse
that's due to the way information is populated, while your instance can see the content, it doesn't actually know its there until you search for it which populates that data (in simple terms anyway)