What it claims to do
The proposal is a markdown file at your root that gives models a clean, curated summary of your site and its most important pages. The intent is sensible: give a model a shortcut to the good stuff instead of making it parse your navigation.
The problem is adoption. A standard only works when the consumers of it agree to consume it, and so far they have not publicly committed.
Why publish one anyway
It is cheap, it is a useful forcing function for writing down what your site is actually about, and if adoption comes you are already there. Treat it as a ten minute job rather than a strategy.
The genuinely useful side effect is that writing it makes you articulate your entity clearly, which you then reuse in your schema and your about page.
What to do instead
Spend the effort on things that demonstrably work now. Allow the AI crawlers in robots.txt. Publish complete Organisation schema with sameAs links. Structure pages answer-first. Earn coverage on sites the models already read. Those four move the needle. A markdown file at your root does not, yet.