Here's a simple use case: human friendly and machine friendly representations at the same path.

For example, let's say I have an api at `/weather` which when requested with the header `accept: application/json` returns something like:

    { "temperature": 25, "unit": "c" }

Now if I open that same path in a browser, I get an html page displaying the day's temperature in a nicely formatted way for humans to read.

Or another case, I want to version an API, such that I can provide different representations based on the version requested. So I look for `accept-version` and if it's `v1` for example I get the `v1` version. GitHub does this, as a real world example: https://docs.github.com/en/rest/overview/api-versions?apiVer...