While you are correct, the reasons for Beehaw defederating from lemmy seems to be something they have to do in order to moderate their own instance. As the technology grows (and hopefully modtools get built - like bots perhaps? and more mods in general), there will be no need to deferederate like Beehaw did. However, to give you an example as to why an instance would want to defederate (essentially, be private), is if it's for an exclusive group of people. A workplace, or a subscription add-on. (think like, subscribing to a patreon, and as part of their offering, you get to sign up for their lemmy (or whatever server) instance). Maybe that isn't how it would work, but in my brain, that's a way it could be used and be beneficial :)

You can see posts/conversations from other instances if the instances are federated(communicate with each other), which most are/do(?). You just have so subscribe to the communities that aren't on your instance to do so. But some instances aren't federated. For example, Beehaw defederated from lemmy.world (and another I forget which) early on due to the open sign ups and moderations issues. I'm on lemme.world, I can go on a beehaw community, I can see the posts, and read through the conversations, but I don't think they can see if I reply to anything( so I don't). But this is where I start to lose my grasp on the technology too, so if anyone more knowledgeable than me wants to chime in...