Why does Activity Pub make servers wrap "Note" objects into "Create" activities?

Question

What I've gathered is that new posts are published by POSTing a JSON-LD Activity Streams object of type Note to an actor's outbox.

{"@context": "https://www.w3.org/ns/activitystreams",
 "type": "Note",
 "to": ["https://chatty.example/ben/"],
 "attributedTo": "https://social.example/alyssa/",
 "content": "Say, did you finish reading that book I lent you?"}

The server will then have wrap it into an activity of type Create.

{"@context": "https://www.w3.org/ns/activitystreams",
 "type": "Create",
 "id": "https://social.example/alyssa/posts/a29a6843-9feb-4c74-a7f7-081b9c9201d3",
 "to": ["https://chatty.example/ben/"],
 "actor": "https://social.example/alyssa/",
 "object": {"type": "Note",
            "id": "https://social.example/alyssa/posts/49e2d03d-b53a-4c4c-a95c-94a6abf45a19",
            "attributedTo": "https://social.example/alyssa/",
            "to": ["https://chatty.example/ben/"],
            "content": "Say, did you finish reading that book I lent you?"}}

I fail to see the usefulness of this, as the wrapping activity doesn't seem to add any useful data to the wrapped note. Worse even, it seems like it might introduce a fair bit of redundancy to the responses (in this basic example from the official page, actor and attributedTo, as well as the 2 to fields, have exactly the same purpose). Is this perhaps done just for consistency, as there are a few other other activity types that are applied to notes, and for newly created posts having just a plain object (or a collection of plain objects) as a response would not fit this way of doing things?

Also, why are other activity types (e.g., Like) able to simply reference notes by id, while Create activities enclose that data directly? Is that required or is there a specific reason for it?

{"@context": "https://www.w3.org/ns/activitystreams",
 "type": "Like",
 "id": "https://social.example/alyssa/posts/5312e10e-5110-42e5-a09b-934882b3ecec",
 "to": ["https://chatty.example/ben/"],
 "actor": "https://social.example/alyssa/",
 "object": "https://chatty.example/ben/p/51086"}

Answer 1

ActivityStreams is a protocole to synchronise datas between different databases and softwares.

Some actions contain the detail (like for Notes) because we can create/update them and an activity stream reader must know the changes to display the good datas to its users.

Some operations like Like have a named cancel operation like Unlike. So we don't need the Create/Update container. and because they operate on existing datas, we also only need the unique ID to the concerned resource.