Some advantages that I can think of:
- Run as-needed without slowing down your primary computer or utilizing local disk space
- You can easily create a large library of ReactOS VMs of various versions and swap easily (or run concurrently depending on core allocation)
- VMs can easily be stopped/started on-demand for release/debug builds
- Library of ISOs (ReactOS builds) can be kept on the server without the need to transfer locally/re-download to build with VB/VMWare
For additional videos, please see my signature. There are some others that cover installing ReactOS to a Virtual Box VM, REAL HARDWARE and setting up the audio drivers, etc.