Computers are designed to do more than one thing, but traditionally many servers were purchased per-role. Mission critical apps would only run on one version of Windows, or apps might not play nice with others or with OS upgrades.
It turns out that one of the apps people really need to run multiple instances of is Windows itself. This is largely Microsoft's fault for bundling every app including the kitchen sink in the OS platform itself. As a condition of using their clean little high-performance kernel, you had to accept a web browser and home-user-friendly userspace.
Little surprise that people are kicking the whole package off of Ring-0 and substituting something like vmware for their $five-figure server hardware.
It's that super-isolation that actually allows multiple apps/roles/data categories to finally share the same hardware.
Interestingly, I view the current state of the world as too much sharing -- VMs are just super process isolation =D