There are a number of tools and systems which realistically require a separate machine to maintain. Examples include distribution installation procedures, package managers, and network services. Commonly, people who maintain or develop multiple versions of these tools keep one physical machine for each version.
UML offers the ability to move all that activity back on to a single machine by putting each version in a different virtual machine. Aside from the logistical convenience of not having to maintain multiple physical machines, there are also the advantages that come from virtual machines being more convenient to manage than physical ones. Resource allocation between them is far more flexible, they are much quicker to boot up and shut down, and they can be created and destroyed at will.
These make it much more convenient to do this sort of development inside multiple UML instances than on separate physical machines.