1. To keep your stuff running that's stuck on that chip. Replacements are being made but not finished for many customers. The article actually covers this for HP's customers. OpenVMS was on Itanium as well with a x86 port underway. SGI was on Itanium but moved to Xeon more quickly.

2. To get benefits that chip has that others don't. Secure64 is the main use-case I know for this. For me, I considered buying some SGI Altix's off ebay that were going for $100-200 each. Reliable chip and server with built-in security features that almost nobody is targeting on top of that. Such an approach has always paid off for me in the past.

https://secure64.com/secure-operating-system/

Because you've already sunk an enormous amount of money into the architecture, and you're looking to defer the day when you have to admit that was a bad idea for as long as humanly possible.