As for why you'd want to do this, there are several Linux apps that are much better -- and with more advanced features -- than their Windows counterparts.
For example, Amarok is a superior alternative to Windows Media Player or iTunes, especially if you ever need to take music off your iPod. Amarok lets you move files in both directions -- onto the iPod and from the iPod onto your desktop, a function that iTunes by itself does not provide. Amarok also has better options for tagging your music than iTunes, and it features integration with Wikipedia and the Last.fm social network.
Web developers and designers could test their pages against Linux browsers like Konqueror or Epiphany. Also, those working in scientific fields probably use a number of Linux-based apps that haven't yet been ported to Windows (and may never be).
Running apps in a faux-native environment will always be easier than dual-booting into a second OS. And aside from the practical concerns, it's just plain cool. Or creepy, depeding on your bias.