Well there's two parts to it. First of all as others have said, it's a hobby. Thankfully my wife understands my hobby and doesn't mind my man-cave (within reason).
For what I do with them, allsorts! I enjoy playing around with IRIX, scripting in Perl, trying to port/compile certain apps (despite my limited knowledge here), I use my Indigo2 for ripping vinyls (though I do usually do the final clean of the audio with Audition on my PC). I don't do a huge amount of productive things with them, but in short, they're a fun hobby.
It's the same reason some people buy classic cars. You know the performance is poor, the economy isn't great and they're way behind with fancy gadgets, but they look cool, they're enjoyable and nostalgic to drive, and tinkering around with them is good fun that some people enjoy.
Systems in use:

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Nitrogen
: R10000 195MHz CPU, 384MB RAM, SolidIMPACT Graphics, 36GB 15k HDD & 300GB 10k HDD, 100Mb/s NIC, New/quiet fans, IRIX 6.5.22

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Lithium
: R14000 600MHz CPU, 4GB RAM, V10 Graphics, 72GB 15k HDD & 300GB 10k HDD, 1Gb/s NIC, New/quiet fans, IRIX 6.5.30
Other system in storage:

R5000 200MHz, 224MB RAM, 72GB 15k HDD, PSU fan mod, IRIX 6.5.30