Virtualization+why?

In computing, virtualization (or virtualisation) is the creation of a virtual (rather than actual) version of something, such as a hardware platform, operating system (OS), storage device, or network resources.^[1]

While a physical computer in the classical sense is clearly a complete and actual machine, both subjectively (from the user's point of view) and objectively (from the hardware system administrator's point of view), a virtual machine is subjectively a complete machine (or very close), but objectively merely a set of files and running programs on an actual, physical machine (which the user need not necessarily be aware of).
The idea of Server Virtualization is to run multiples servers on one physical machine.This is basic of running the Virtual Private Servers. So why we need Server Virtualization? Just it's because this technology provides High Availability, Economy and Flexibility.. It also thinking about costing in any system developments.