To quote Micha:

Let's say you have a very long document and the syntax error is outside the visible area. It might now be unclear to users why the manually triggered action does nothing.

RustRover does show an error when running rustfmt failed because of a syntax error and I find this useful information.

I also think this is a useful indicator but might be difficult to achieve for a language server. This can be achieved at least for code actions using the CodeActionTriggerKind but for other requests like formatting and workspace command it's difficult without having full control over both the client and the server.