You don't _have_ to test anything, just like in any software development. But now you can, and you can do it in a side-effect-free environment, on CI push, instead of realizing you made a typo when the CM kicks in, or worse, when your system starts crashlooping on startup because of a configuration issue.
I, personally, would rather prefer straightforward static configuration and very well written documentation on why certain configurations make more sense than others. That way, I know how and when to change something in the configs.
I absolutely do not share the view that "tests are documentation", because they do not communicate the most important thing in software engineering - the intent and reasoning behind a certain design.
This would be a sign of a program that allowed input (configuration, in this case) that was unsafe. That is completely a different matter to whether a programing language is suitable for defining configuration. I suspect that many an application has been written that assumes that because it's consuming "static" configuration that is a simple text file, or a structured data file, that the input is completely safe. (SQL injection can come from any direction.)