Often the 2nd PC/FS is a machine dedicated to storage - typically for multiple users, and typically is running a Server-grade OS. In my case, I have a PowerEdge 1420SC that is overkill for just FS usage, so I made it my DC/DNS/DHCP/IIS as well. I also installed Hamachi on it so I can 'VPN' into it and accessing it quasi-locally when @ work. The great part about this is that I don't have to mess with my main rig or dump additional power requirements on it to support all the drives/etc, or any additional cooling either.
My FS is used to store my entire Profile for both my wife and I (roaming enabled), and all in RAID5. Many people end up swapping out a FS for a NAS, since they are 'virtually' the same thing (if using explicity for external - typically RAIDed - storage).