Another example:
I own a Mac and write some software that can be used with either a MySQL or Microsft SQL database. I obviously need to test and make sure it works on MS SQL but I don’t have a Windows server. So I create a VMWare virtual machine of Server 2003 and SQL Server 2005 and can now test everything while sitting at the same computer and not needing a separate computer.
Also, since I really don’t know what I’m doing when it comes to managing a Windows database if I break something I simply delete the VM image and grab another fresh working copy that I have saved.
You can pretty much do anything.