Why do servers have 2 psu?

Solution 1:

You have two power supplies for redundancy. Assuming they are hot-plug, if one fails you can replace it while the system is still running.

You could also run to separate servers in your data center and connect each power supply to a separate circuit. So if a something breaks one circuit your server would still be running.

Solution 2:

As two power supplies are for redundancy, a single PSU should always have enough capacity for the whole server: you could leave the other one unplugged, if you wish.

Redundancy for environmental problems comes usually prior to redundancy for PSU failures, as they occur more often, and a failing PSU may also harm other components. The units could have separate power sources altogether, or separated UPS solutions, so that a failing battery on a single UPS wouldn't cause power outages.

And for anyone who even thinks about it: they are NOT for moving a running server to another place. ;)