Wouldn’t this also be possible with plain sockets tho?
You keep using this phrase. Given time and money anything is possible. Technically we don’t need to use http - every server could implement their own standard using raw sockets. You then could download a simple client library for every site!
With a well defined dbus interface your application can talk to any number of applications that implement that interface. Even those you didn’t know about it at time of development. It provides a structure for ipc other than “go fetch libblah” and also “libblarg” and “libfloob” and read all of their docs and implement each one separately.