I looked around and struggled to find out what it does?
My guess would be that it notifies you of when new posts are made to communities you subscribe to. But that sounds like a lot, so I’m really not sure.
Otherwise, is it me or does the wording here not speak for itself?
Ah right. Because I think you could build it into a frontend or app and anyone using that app would be able to see a feed of joined up communities.
I’m not sure how a different implementation would work. There is a lot of public/private key stuff happening in ActivityPub so you can be sure that the federated posts are actually coming from the community you subscribe to and the users it claims wrote them.
If you work it out I’d be interested to see!