What is D-Bus practically useful for?

dbus is supposed to provide "a simple way for applications to talk to one another".

But I am still not sure what it is useful for, practically. I have never seen a situation where dbus is useful, I only see warnings that some dbus component has experienced errors, such as when I start terminator from commandline (so that I can see errors):

Error retrieving accessibility bus address: org.freedesktop.DBus.Error.ServiceUnknown: The name org.a11y.Bus was not provided by any .service files

I got rid of the above error by adding NO_AT_BRIDGE=1 to /etc/environment. I have no idea what that does.

Almost all gui applications seem to be linked with dbus. Some allow to be started without dbus, ie:

terminator --no-dbus

I see no difference in behavior. What is supposed to stop working, when terminator is started without dbus?

Also, I have tried disabling various dbus components to see what stops working:

I have deleted /etc/X11/Xsession.d/95dbus_update-activation-env just to see what happens. It contained the following code:

if [ -n "$DBUS_SESSION_BUS_ADDRESS" ] && [ -x "/usr/bin/dbus-update-activation-environment" ]; then  # subshell so we can unset environment variables  (    # unset login-session-specifics    unset XDG_SEAT    unset XDG_SESSION_ID    unset XDG_VTNR    # tell dbus-daemon --session to put the Xsession's environment in activated services' environments    dbus-update-activation-environment --verbose --all  )fi

Everything works the same, as far as I can tell. What was the purpose of the above script?

In what situation would it be useful for my applications to talk to each other via dbus?

Are there applications that don't work without dbus?

My system is Debian Buster, and I am using plain openbox environment (without any desktop environment such as Gnome or KDE)