There are so many ways virtualization can provide benefits in an enterprise environment.
Consolidation - This is the number one benefit for most enterprises. The ability to combine different system workloads on a single physical box allows more efficient use of physical resources. It also allows an entire rack of hardware to be consolidated down to a single physical system. The savings on rack space, cooling, power, etc can add up to be quite significant.
Scalability - Using an enterprise level virtualization product like VMware provides the ability to add additional host systems seamlessly. That instantly increases resources, which can then be used by guests immediately.
Reliability/Redundancy - With high availability options and fault tolerance, hardware failures become a minor inconvenience instead of a massive outage. Automated load balancing (with additional features) can keep redundant servers on separate physical hosts to avoid outages (ie, VMware DRS rules for separating Active Directory controllers).
Simplified Management - Enterprise level virtualization generally include fantastic management tools. VMware includes tools to not only manage the hosts, but will also manage the guests to some level (OS patches at least).
I talk a lot about VMware - primarily because that's what I'm familiar with, but also because its what we generally recommend. (We usually recommend it because it has better management tools in our opinion.)