Virtualizing servers helps pack more compute resources into fewer physical boxes. Streamlining an enterprise in this fashion with a virtual server infrastructure can reduce the costs of buying, updating and maintaining all that server hardware during the inevitable technology refresh. Less hardware to power up means you're using less electricity and saving on operating costs. Fewer servers also means you'll reduce the impact of these machines on the environment.
Going virtual requires forethought and consideration; there are other costs besides the infrastructure to worry about such as software and licensing. But the ability to provision computing resources allows IT shops to achieve scalability without making it necessary to buy a number of new machines. This makes virtualization a powerful tool in your cost-cutting arsenal.
Hardware, operating system and virtualization technologies have evolved to the point where systems administrators have been given tighter reins on power use in highly virtualized environments. Most hypervisors can monitor and tweak power use depending on the needs of the enterprise.