Does anybody know why dbus exists? I've been wracking my brain trying to come up with a usecase for dbus that isn't already covered by Unix sockets.
You want to remotely control a daemon? Use sockets. You want the daemon to respond to the client? Sockets. Want to exchange information in json? plaintext? binary data? Sockets can do it. Want to restrict access to a socket? Go ahead, change the socket's permissions. Want to prevent unauthorized programs from pretending to be someone they're not? Change the permissions of the directory containing the socket. Want network transparency? That's why we have abstract sockets.
Plenty of well-established software uses sockets. Music player daemon uses sockets. BSPWM uses sockets. Tmux uses sockets. Pipewire uses sockets. Dhcpcd uses sockets. Heck, dbus itself relies on sockets!
For developers, using sockets is easy. I once wrote a program that interfaced with BSPWM, and it was a breeze. Dbus, on the other hand, not so much. I tried writing a Python script that would contact Network Manager and check the WiFi signal strength. Right off the bat I'm using some obscure undocumented package for interfacing with dbus. What is an introspection? What is a proxy object? What is an interface? Why do I need 60 lines of (Python!) code for a seemingly trivial operation?
So why do some developers decide to use dbus when they could just use unix sockets and save a lot of hassle for themselves and others?
With pipes/sockets, each program has to coordinate the establishment of the connection with the other program. This is especially problematic if you want to have modular daemons, e.g. to support drop-in replacements with alternative implementations, or if you have multiple programs that you need to communicate with (each with a potentially different protocol).
To solve this problem, you want to standardize the connection establishment and message delivery, which is what dbus does.
With dbus, you just write your message to the bus. Dbus will handle delivering the message to the right program. It can even start the receiving daemon if it is not yet running.
It's a bit similar to the role of an intermediate representation in compilers.
This reminds me of QT's signal/slot system. I.e. instead of calling functions directly, you just emit a signal and then any number of functions may have the receiving slot enabled.
Lot's of similar systems in other frameworks too I'm sure.