That is precisely how we use it, intranet only. Our work is primarily project-based, sometimes with projects under NDA, so we have a Projects category and tend to create access-restricted subcategories of Project for individual project work. We also have broader topics that cover thematic issues that span projects, such as “Health Care,” “Criminal Justice,” etc.
Sometimes we use wiki posts at the top of topics. For instance, we have dozens of internal prototypes and tools running on various ports of a dev machine. We use a wiki post in a pinned topic to track which ports are in use for which project. In general, I find the wiki posts to be less useful for larger discussions.
Overall, Discourse is a huge hit at my company.