|These several OSes can run at the same time.|
All OSes can use all the hardware, connected to the host.
If you have e.g. 3 servers. One is running Windows Server 2008, second Sun Solaris, third is running Novell Netware.
Each of this machines will have average utilization of lets say 20 percent.
If all these OSes will run on only one machine, the machine runs more efficient.
It's also much easier to move the OSes to another machine, if needed in worst case, when server hardware died.
These are only a few reasons for virtualization.
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