But there are also users who read them if they think that some particular behavior is puzzling. Then they check the spec to see if this stuff really is supposed to happen or if this is something which wasn't properly specified.

For example, there was a 50:50 split when it came to the behavior of SVG masking. As it turned out, that part of the spec was just confusingly written and the compliant behavior was actually really silly. The spec was changed and now every browser does the same intuitive thing.

Without this "central authority" there wouldn't have been a way to get this fixed.

Dart is another interesting example. They had a specification since the beginning and every part of the ecosystem follows it. This way, different teams (and even different companies) can create all kinds of tools which all have the same precise understanding of the language. Naturally, this also helps with identifying spec issues.

Without a specification, you have to reverse-engineer the behavior and you might end up being forced to replicate some really nasty crap. Some of JavaScript's warts were set in stone this way.