The institutional memory, and the reason this site exists in the shape it does. Confluence is excellent at capture and poor at retrieval — the page you need was written, by someone who has left, under a title you would never guess, in a space you do not have permission to search. A garden with stable URLs, a schema and a search index is the answer to a specific frustration with wikis, not a rejection of writing things down.
Where it is genuinely strong: a decision recorded next to the ticket that prompted it. Architecture decision records in a repository do the same job closer to the code, which is the trade — Confluence reaches people who will never open a repository.