Because it means a VMM can start a VM, wait for the X_SYSTEMD_UNIT_ACTIVE=ssh-access.target sd_notify() message and then immediately connect to the VM via ssh-over-AF_VSOCK. Reliably, without wasting time, without retries, without network config and setup, without preparing the guest much (well you do have to add sshd to it), just like that.
And that's all for this time. We still have more to cover, hence stay tuned.