The sales pitch is this: Pool your computing needs with those of dozens of other customers and you won't have to keep your own programmers and troubleshooters on the payroll.
In essence this means that the servers in a data centre are turned into a pool of computing power to be tapped into as needed rather than being used individually.
The ultimate goal of virtualisation is to make a data centre, or even several of them, look like a single pool of computing, storage and networking resources that can be allocated as needed.