Well my (shallow) understanding is that some entities are required to buy bonds of certain rating because of regulatory reasons or because that's what they promised investors. Other entities may not have such requirements but do rely on the rating because they don't have the resources to check the creditworthiness. And yet other entities may disregard the rating entirely and solely put their faith in their own analysis.

If I bought bonds myself I certainly wouldn't trust the rating. I'd only use it to serve as an indicator of how likely I can find a "greater fool"

So you have zero bonds in your portfolio? Directly or through funds (mutual/ETF).