While pursuing the 100% virtual mark you once in a while run into trouble with legacy hardware where there is hardware directly attached to the physical server you are going to virtualize. If it is USB equipment you can use USB Anywhere devices to make it happen, but if it is serial port connected equipment it gets harder to tackle that issue.
For instance you want to virtualize a building management system server with reading equipment connected to the serial ports. Is that possible? Yes you can!
By connecting physical serial ports over the network with a virtual machine you can break the dependency of the physical layer and the OS layer. By solving this puzzle you can protect the organization against legacy hardware failure and/or from software that nobody knows how it is installed anymore.