Good post by George Crump (thank you)
Before hypervisors like VMware, Hyper-V and KVM came to market, data centers had few options when it came to managing the growth of their server infrastructure. They could buy one big server that ran multiple applications, which, while it simplified operations and support, meant that one application was at the mercy of the other applications in terms of reliability and performance. Alternatively, IT professionals could buy a server for each application as it came online but this sacrificed operational efficiency and IT budget to the demands of fault and performance isolation. Until hypervisors came to market, the latter choice was considered to be the best practice.
Read on here