it just codifies the “anyone can decide” thing, which doesn’t need codifying.
As I understood the idea, it is about comparability, and readability, through unification.
Each instance can decide for itself, and most align with what I like. But I still have to figure it out for each instance individually. By finding where they post that information, understand how the information is structured, note they did not specify something the others did specify in their prose, and so on.
If we codify what key-value pairs make up this information, and where this data should be displayed, comparing becomes much easier.
It also allows automatic comparison. Imagine I could filter in instance browsers for policies which I do or do not want my instance to have.