What are some reasons to run Windows on a Mac?
Games
Internet Explorer: If you are a web developer and want to test your website in IE (which you have to).
Also some corporate intranet applications work only on IE(6/7/8 compatibility mode).
Microsoft Office: Yes, there is a version for the Mac, but some prefer the Windows version (I remember reading sometime before somewhere, (unable to find the link) that the Windows version is better). Also, people like me who have a Windows PC (my wife's) and a Macbook Pro at home, I can buy one Office Home and Student edition (it comes with licence for 3 installs) and use it on both.  
Learning Windows specific technology. For instance I want to learn C#. C# is best learnt in Microsoft Visual Studio (IMHO). 
Games
Games
Games


There was something else too... oh yeah, games.