Redundant power supplies

Most of my boxes have dual power supplies.  Most
don't give me an option as to how they are used/configured;
one shits the bed, the other is there to cover the load.
I have always *assumed* they were configured to SHARE the load.
I picked up another box that gives me the option of NOT operating
them redundantly (what the hell does the "extra" one do, just
sit around??).  And, when in the redundant configuration,
allows me to choose which is the "primary".
This suggests one is carrying the load and the other is switched
in (even if passively) when that one fails.
Is there any advantage to this over a "sharing" configuration?
And, why would I ever want to *disable* PFC?