I found this link for a group that is converting fedora RPM's to run on S/390 hardware. Wikipedia tells me that the S/390 was made by IBM as a mainframe computer in the 1990's (and run out of service before 2000). So I have two questions: Why would you run a "standard desktop" OS on a (deprecated) mainframe? and are there any practical applications for a 1990's mainframe nowadays? The site seems to be pretty active, so I feel I must be missing something in its purpose.
Like most distributions, Fedora is not really only a 'desktop' OS - it is a 'server' OS as well. It depends on what packages you install since in the repositories are both desktop and server applications available.
Thus, I guess that you want to use Fedora on S390 for server stuff (mainframes are advertised for their IO performance/throughput, batch processing, right?).
As with other vintage hardware there are probably three major reasons to run Linux on it:
How active the Fedora s390 project is I don't know. I mean, some packages you care about you have to port, but other portable software just need to be recompiled and once a buildserver is configured for yet another architecture, new incoming packages are automatically built.
Wikipedia also notes: