Changes to a declarative operating system, such as NixOS, are atomic. This allows for easy experimentation and rolling back to older configurations.

For example say you install gimp for editing photos. Normally you'd just install it using command line or a clickidity gui program. But say you don't like it. Maybe it causes an issue. Then you have to uninstall it again. You are applying yet another action to the same system. That system is mutable, or modifiable, and that introduces some extra complexity.

With NixOS you can simply roll back to the previous state you had before installing it. It also doesn't have to support stuff like uninstalling. The downside is that it likely uses a bit more resources when changing configurations.

This also applies to stuff like user management, services, e.g. a webserver.

Any experts correct me if I am wrong, I haven't tried any of these systems yet.