Fun, basically. My dad worked at the VR centre at a local university back in the 90s, I went in from time to time during the school holidays and fell in love with the SGI machines there (beautiful and impressive things, coming from a world of beige PCs as I was back then). Several years later, I got my hands on a couple of the older student systems when they were being let go cheaply.
Got bored with them mid-2000s and popped them in the loft, but caught the bug again after moving house a few years back (and finding them in the loft). I've since added slightly to my collection to acquire my most "sought-after" machine (the Indigo2). It's just pretty cool to play around with a system that was worth around £50,000 when new - especially with their connection to the movie industry, plus playing with pure old-school Unix is also quite fun.
Now that's a proper desktop.
Do you do anything fun with it?
A few odds and sods! IRIX is an interesting OS to customise and play around with, and I've done a bit of work with porting a couple of apps to IRIX, mainly rdesktop (open-source Remote Desktop client), plus also some work a while ago with OpenTTD. Other than that, the Indigo2 sounds great for music and can rip vinyls quite nicely, playing the odd retro-game like Doom or various ScummVM games is also fun.