

Lemmy Federate doesn’t do anything special. It searches and follows communities just like normal users do. The only difference is that it does this from every (registered) instance, for almost every community. The reason I didn’t explain it in more detail is that it has evolved into an admin tool rather than a user tool. Generally, admins know what this tool is for.
However, I can explain a few things that will be useful for you outside of the “All” tab:
Discoverability
Lemmy Federate enhances community discoverability by allowing users to search communities from their instance. Instead of relying on an external site, you can discover new communities while staying within the context of your own instance.
Crossposts
Let’s say that technology@lemmy.world is federated in your instance, but technology@lemmy.ml is not. In this case, you won’t see crossposts from technology@lemmy.ml sharing the same link as technology@lemmy.world.
Initial post count
When a community is pulled for the first time, only a certain number of recent posts are pulled with it. If you are the first to follow a community you are interested in from your instance, you will not be able to see all posts from that community.
In order for an instance to follow a community in another instance, both must be registered in lemmy-federate.
If an instance has the “auto add” feature enabled, newly created communities will be added automatically within 1 to 5 minutes. This process is automatic. No action is required from an admin/moderator/user. You can see this from instances page.
If “auto add” is not enabled, any user can add that community manually. If not added, lemmy-federate will not work for that community.
Off topic, no need to apologize this much :)