I have a few test servers that I wanted to be virtualized but have no access to the campus network or Internet. I also wanted to be able to give those servers access to the Internet or campus network easily for whatever reason. Some good use cases are test environments for domain controllers, SCCM upgrades, you name it.
In vSphere, I created a new vSwitch that does not have access to any physical network adapters. When a particular vm needs Internet access, I can simply change the network settings on that vm's network adapter.
Pretty simple stuff.