The use case is using Discourse as a fuller AP node. There are many easier ways to post content to AP (eg use category RSS feeds and Zapier or Buffer) — but developing fuller AP capability can only be done as a plugin/integration.
Article is the ActivityStream type that is meant for full articles. Depending on the client interface, it will show a preview and then a click through to show the entire article inline (much like content warnings, but read more).
Note is the micro-blogging type.
By having full Article posts, people can directly read / boost / reply in their AP clients.
And of course, it would be interesting to hear about your roadmap if you’re going to follow a more Microblogging AP instance, or go in a federated forum direction like Lemmy or Kbin, especially given the recent Reddit news.