D-BUS contains some tools that require Xlib to be installed, those are in this separate package so server systems need not install X.
-rwxr-xr-x 1 root root 12676 sep 30 10:54 /usr/bin/dbus-launch -rwxr-xr-x 1 root root 6980 sep 30 10:54 /usr/bin/dbus-monitor -rwxr-xr-x 1 root root 28932 sep 30 10:54 /usr/bin/dbus-viewer