Imagine production servers which are deployed fully automatically and configured securely such that none if the directories are writeable except public and private storage areas.
However, the config sync directory might be outside those writeable areas which helps to keep an eye on every change to the configuration on live sites. All the configuration is most likely also stored in and will be overwritten the next time the site is being deployed.
This detects changes to the configuration system and exports those changes into a configurable directory of your choice. In addition to that it is able to fire a POST request with parameters to a http service that you configure which can then react to those configuration changes.