In what context would a user discover parts of a REST API dynamically?

In the simple (albeit niche) case, a UI could populate a list of buttons based on the URIs/verbs that the REST API returns. So the UI would be totally dynamic based on the backend - and so, work pretty generically across REST APIs.

But for a client, UI or otherwise, to make use of a dynamic set of URIs/verbs would require it to either look for a specific keyword (hard coding the intents it can satisfy) or be able to semantically understand the API (which is hard, requires a human).

Oddly, all this stuff is full circle with the AI stuff. The MCP protocol is designed to give AIs text-based descriptions of APIs, so they can reason about how to use them.

The simplest case, and the most common, is that of a browser rendering the HTML response from a website request. The HTML contains the URL links to other APIs that the user can click on. Think of navigating any website.