For servers hosting many actors, delivery to all followers can result in an overwhelming number of messages sent. Some servers would also like to display a list of all messages posted publicly to the "known network". Thus ActivityPub provides an optional mechanism for serving these two use cases.
Comments
From https://www.w3.org/TR/activitypub/#outbox-delivery
You don't necessarily have a sharedInbox though, I think?
Servers are not required to provide a shared inbox, but if they want to enable batching behavior they can. Not sure if that's what your asking?