Why shouldn't I manage my configs by hand, as I've always done?

The problem isn't managing any single config, for a single device or service; that can clearly be done by hand, and it either works or it doesn't.

The problem is managing multiple related configs across multiple devices and services. How do you ensure that those configs are consistent and complete across all those devices and services? If you do it by hand, you're probably going to make errors, which in turn are going to make your network less reliable and harder to scale. On the other hand, if you generate all those configs from a common model of your network using Netomata's tools, those configs will be consistent and complete. And even if you make an error in setting up the templates that the configs are generated from, it will be much easier to fix the template and regenerate the configs than it would be to fix all the individual configs by hand.