Mostly, that not every instance has the same policy.
You can always join a place where the credo is "we connect with everybody", while people who want a very filtered experience can join places that are a lot more nonchalant about blocking other instances. A lot of people probably do (and want to be in a place that does) something in the middle.
You could even have multiple accounts on different instances that you use in different contexts. Most mobile clients support this more or less well, though the default web UI does not.
You can always join a place where the credo is "we connect with everybody", while people who want a very filtered experience can join places that are a lot more nonchalant about blocking other instances. A lot of people probably do (and want to be in a place that does) something in the middle.
You could even have multiple accounts on different instances that you use in different contexts. Most mobile clients support this more or less well, though the default web UI does not.