The GConf database also stores useful meta-information such as the application that owns a key, when the key was last changed, etc. Data change notification service. If configuration data is changed, all interested applications are notified. The notification service works across the network, affecting all login sessions for a single user. Without GConf, a custom protocol would have to be invented for each preferences setting, to notify embedded components of changes to user preferences.
Notification is also useful if multiple application instances are running.
The client API is very abstract, which allows us to change its implementation at a later time without a big headache. Because a good implementation is a complex problem, this is important. I also like to think the API is simple to use.