Because I want to run different applications that only are available, or work best, in different OSes. For instance, Citrix GoToMeeting is only available for Windows, iPhoto is only on MacOS, etc.
I've always got half a dozen windows open to different tasks -- it's how I work. Email in one, a few browser windows on different sites, a Word doc, a spreadsheet or powerpoint, GoToMeeting, Skype, a text editor, a few xterms to the Unix boxes, etc. Back in the bad old days I had to either be dual-booting all the time (lost productivity) or have an array of machines on my desk and a KVM switch. Using VMs (virtual machines) is the perfect solution.
Your response has brought up another question: how do you run two operating systems at the same time on the same computer? Have only tried running more than one OS on a single computer recently (a computer with Windows broke down, and some freepers suggested that Linux be used), and to get on Linux, it was selected in the boot menu (although the Puppy Linux desktop did have tab on the bottom with Windows in it (but it didn't work)......). Is it similar to having multiple browser windows open at the same time, and there's some way that toggle between OSs. Curious.