There are two ways to run the Libvirt daemon, which are unprivileged and privileged aka system. You are using unprivileged mode, the parent is using system mode which is more powerful and provides better isolation but does hide stuff in /var.

For example, running QEMU as its own user and using PCI passthrough is only possible with the system daemon.

You also need the system daemon to set up bridged networking, though the unprivileged daemon can use it through a setuid helper.