In pictures: Taking apart Hewlett-Packard's Prolia...
The spare power supply slot allows the HP DL380 G5 to survive a power supply failure without having to be restarted.
The IO ports can be used if external devices need to be plugged in. The DL380 G5 still provides PS/2 keyboard and mouse ports for those who require them, as well as serial and USB ports.
In pictures: Taking apart Hewlett-Packard's Proliant DL380 G5 server