This is to have a simple built-in support for a plausible-analytics key, like we have for google-analytics.

I say simple because what this does it only accepting the HTML snippet that a user is supposed to get from its plausible dashboard: https://plausible.io/docs/plausible-script

website:
  title: "Plausible Custom Script Test"
  plausible-analytics: |
    <script async src="https://plausible.io/js/pa-TESTCODE123.js"></script>
    <script>
      window.plausible=window.plausible||function(){(plausible.q=plausible.q||[]).push(arguments)};
      plausible.init()
    </script>

It seems there are ways to use a generic https://plausible.io/js/script.js and pass domain as attributes. However, this is not documented and could be harder to maintain.
Also the user workflow will always be to go to its plausible dashboard to activate, and so copy pasting the snippet is easy enough.

More complex config would allow something like google-analytics

website:
  plausible-analytics:
    domain: example.com
    ... 

but again, not well documented and so probably not easy to maintain and test.