No engine has committed to reading it, and anyone presenting it as a ranking lever is overselling. What it does cheaply is remove ambiguity: a short, human-readable statement of what the site covers, how its data should be interpreted, and where the important sections live. This site publishes its own for exactly that reason.
Treat it as complementary to robots.txt and the sitemap rather than a replacement for either. It costs an hour to write and needs updating when the site's structure changes materially — beyond that, expectations should stay modest.