The relationship is very synergistic. Suppose we have two equal Tier 1 storage boxes offering 1 terabyte (TB). I can create a pool of storage between those two boxes and have up to 2 TB available. Since it's a fluid pool of storage, capacity planning becomes much easier, especially if the storage virtualization software on both boxes supports thin provisioning. For example, I can over allocate to the application and have less physical capacity than I've allocated. So the applications can be quite satisfied, but at the physical level, I'm getting very good capacity utilization. As the storage demands approach the physical capacity limit, an alert can inform me that I need to add capacity. That capacity can be added at a much more granular level, which offers significant capital expenditure advantages.
Check out the entire Storage Virtualization FAQ guide.