Virtualization can save money on an ongoing basis. As already mentioned, less maintenance and upgrade cost because of less hardware. Even more so, when done correctly, it can keep apps and databases up and running despite hardware failure on a server, which serves much the same purpose as clustering/mirroring, but without the "we have a server that just sits around doing nothing but prepare for disaster". Keeping apps and web pages and databases up and running, instead of allowing for downtime, can save a company a lot of money in terms of preventing lost opportunities.