I first began to use virtualization for software development. Procuring development machines and building out test environments that reasonably mirrored production was expensive. Even when money and resources weren’t the issue the lead time involved in assembling enough hardware to create an integration environment for new software was.
First generation server imaging and duplicating tools like Ghost helped but it was still a nasty and tedious process to build out a server farm complete with domain controllers, database servers, email servers, load balancers, etc. Virtualization software changed all that.