Is an Archive Mailbox just a regular mailbox? Or is it special? Apple Mail 9.3 has an archiving feature. I'd like to better understand what this feature does, how it works, and why someone would want to use it. Unfortunately, I'm having a hard time finding detailed documentation about it.



Here's all that I can find in the documentation:  


  
  "You can store messages in an Archive mailbox so you can quickly find them later when you’re ready to take action. [...] Mail creates an archive mailbox in the Mail sidebar for each account whose messages you’re archiving, and moves the messages you selected there. The messages remain there until you delete or move them."
  -Apple Mail Help  
  


Find them later? Well, heck, I can find messages later in my Inbox or anywhere else I decide to store them, right? Does an Archive provide an improvement in that regard?

Other mail apps/services also have Archive mailboxes, which makes me think that they are fairly standard now.


  
  "If you want to clean up your inbox without deleting your emails, you can archive or mute them."
  -Gmail Help 
  "This (system) folder has no additional functionality other than a handy storage place for mail you no longer consider to be current. Mail placed in this folder is automatically marked as read. You may wish to set the folder to Auto-Purge, so that old mail is automatically deleted from this folder."
  -Fastmail.com Help
  


So, below is a description of my understanding of how Archive mailboxes work in Apple Mail. Can you verify that I'm correct? Or could you expand upon this knowledge, or correct any misunderstandings?


In Apple Mail, an Archive mailbox just a regular mailbox, similar to the Inbox, Sent, and Drafts mailboxes. 
When an item is moved to an Archive mailbox, the message will remain on the server where the message originated. It will not move it off the server to a local location, i.e., to On My Mac.
There's not a performance benefit or storage savings to storing items in an Archive. (Because, I presume, Archives are synced via IMAP like other mailboxes, and items therein are not compressed or anything.)
Whereas some mailboxes in Apple Mail can have "behaviors" associated with them, this is not true for an Archive mailbox. Example: In Preferences, I can specify whether messages in a Junk mailbox should be stored on the server and/or deleted automatically after a certain period of time. 
If I want to archive messages to On My Mac or a local location, I'd have to manually create a folder and move the messages there; I couldn't do so by using the Archive command.
The computer can track thousands upon thousands of messages no matter what mailbox, so you are right - this is purely a human affordance.

Setting aside any other mail service (since this is about Apple) - the archive is there purely to be a quick parking place for a message you don’t want cluttering your inbox. Most people that have used mail for a while will decide that inbox zero is a good thing so you need a place to move messages you’re not ready to decide to reply / delete or file.

There’s nothing fancy about archive - it’s purgatory for messages and I wouldn’t use it unless you find it helps you out organizationally. I think the list of premises you have stated seem to line up and you’re not missing anything special or dramatic about this topic or the design of Apple mail app.