For years I've wondered why people buy Mac Pro systems at all. As I wrote in early 2009, performance tests show an iMac is faster than a Mac Pro, so given the fact that a Mac Pro system is much more expensive than an iMac, and doesn't include a monitor, why would anyone buy a Mac Pro?
One reason I've learned that people might buy a Mac Pro is that an iMac quickly turns into an indoor heating system under certain conditions, including watching video online, and watching DVDs. Whenever I watch a movie on Hulu, a baseball game on MLB.com, or a DVD, my 24" iMac gets very warm, and the internal fan(s) fire up to avoid burning down the apartment down, and the smell of burning plastic (or something else) begins wafting through the air.
So, if you're wondering why you'd buy a Mac Pro instead of an iMac, well, that's one reason.