I've been hosting for 20 years, but I've never really gone after a market wider than existing IT clients.
So I'm in a place where I've got great bandwidth in my office, and a cluster of fast machines, and lots of networked storage that I've been using to host online forums. But I've also got a VPS that's working well, and expanding to more VPS systems makes sense in the short term instead of going back to colo'ing equipment again.
So I have empty servers that can host WordPress as easily as forums, and I have VPSes that can be used for WordPress but probably not forums or anything that required any real amount of storage.
So Servers A and B and C, here in my office, can handle anything. So I'd like them in a Forum Hosting group.
The VPS and future VPSes? Managed WordPress hosting.
But there's no reason I can't put WordPress on Servers A and B and maybe C here in my office as well. Hell, the hardware is already paid for...
That's all. Some servers are suitable for all clients, some a more limited set, and being able to add servers to more that one group simplifies that.