Knowledge is genuinely good at the structure: writing, organizing, and surfacing the right article to the right case. What it has no way of knowing is whether the article's content still matches reality outside Salesforce. A troubleshooting article referencing a product's current firmware version, a warranty article referencing a policy that changed last quarter, a pricing FAQ that hasn't been touched since the last plan update, none of that updates itself just because the record it describes changed somewhere else.
That's the gap custom work closes: connecting the systems that actually hold the current truth, a product catalog, a billing system, a policy engine, so an article's content, or at least the specific fact inside it, stays accurate without someone remembering to go edit it by hand every time something changes upstream.