dbus handles a lot of cases where you have an unprivileged process like a GUI widget communicating information to a root process like a network manager. Because it's centralized and everything flows through the dbus process, a lot of it is about policy enforcement— ie, these methods can be called by this user or group, these other ones by this other group, etc.

Examples of the configuration which controls this kind of thing:

https://github.com/mungerd/vncbox/blob/master/data/dbus-conf... http://git.kernel.org/cgit/network/connman/connman.git/tree/...