Joining a Mastodon (or GNU Social) instance like this one doesn't limit you to seeing only the users on this instance, because this instance automatically federates with other instances.
What that means is that when users on this instance interact with users on remote instances -- reply to them, favorite their posts, boost their posts -- those remote users become part of the "federated timeline" of this instance.
So the more users on spanner.works talk to other people in the federated network, the more interesting the federated timeline becomes. And it becomes interesting in a way that's similar to how interesting our own users are in the first place.
The local timeline allows users on this instance alone to talk amongst ourselves -- but it also shows when our users interact with remote users, so if you see a conversation one of us is having with a user on, say, witches.town, you can chime in.
As for posting, note that you have fine control over the privacy of each post using the globe icon (including DMs) and you can embed a "content warning" in any post, which hides everything below the CW until someone clicks. This gets used for everything from hiding the punchline of a joke to hiding triggering content.