In the specific case of configuration files, the danger is easy to underestimate because configurations are often considered a minor, easy,low value part of an application.

For more complicated configs, a common approach I see is that the “config file” is a program that generates the actual configuration data. If you just need to parse and introspect the config data, you fetch the build output and parse that, which does not have side effects.

But large systems are not like this. There is no single configuration instance, configuration is pulled from all kinds of places, and each part of the system sees a different subset of configuration. And very importantly, this changes over time. The impact of a single change in some place becomes very hard to predict. Making the language total is one way we can limit the impact of configuration changes. Others that you mentioned are commutativity and the lack of side effects.

Also, being total means you can freely evaluate CUE outside other contexts than simply program configuration. For example, you can do configuration testing independently of deployment, or you can do large scale automatic refactoring, you can inspect the system in arbitrary ways, etc.