It's a tradeoff. Having separate hardware more fully emulates a cluster you'd see in the real world where you have to deal with cross-machine network boundaries

It's a tradeoff. Having separate hardware more fully emulates a cluster you'd see in the real world where you have to deal with cross-machine network boundaries and potential networking limitations. Sometimes those can be masked when you run multiple VMs on one host, especially if you rely on whatever VM management tool's automatic networking configuration sets up. The other thing a single machine can't easily emulate is having a multitude of CPU cores available across different machines, or the ability to have swapping on one application instance not take down the entire machine (well... that could be mitigated somewhat but still).