In dbus, it seems the feature is intended for two processes to know they can access the same shmem and other system resources. I'm struggling to understand in which circumstances would that be useful.

Creating an excuse for creating a machine-id to associate with network traffic. Sometimes, it is enough to have a plausible enough sounding reason to write down on paper, but you have to look at what something actually is. Any red blooded hacker knows there's what a tool is meant to be used for, and then there's what it can be used for. Less is more.