I have used both VMWare Fusion and Parallels Desktop.
I agree with Peter in that Parallels is a better program than VMWare.
Since moving to the Mac back in 2009 I’ve found myself running Windows less every year. Part of that is because more apps are web-based but also there are better apps on the Mac.
With Parallels and also VMWare, you can easily run Windows apps side-by-side with your Mac apps
I run Windows 8.1 in BootCamp and also in Parallels, primarily so that I know how it works. I plan on upgrading to Windows 10 when it is released at the end of July.
Parallels makes it super-easy to install other “guest” operating systems, like Ubuntu Linux, Windows, and even other versions of OS X.
Virtual machines can be very useful for developers who want to test their apps in other browsers or environments. They can also be useful for “muggles” who wants to switch to the Mac, but still has Windows programs which aren’t on the Mac yet or need to use web sites which need to run on Internet Explorer.