In pictures: Taking apart Hewlett-Packard's Prolia...
The UID light is similar to the one on the front. The back light allows users to easily identify a server when going between the back and the front of the rack.
The iLO2 port is used to remotely access the server. It provides SSH (secure shell) and HTTPS access to diagnostics, and allows remote access to a virtual serial port for those want console access.
The back access ports are similar to the ones on the front of the server. This allows you to plug in a monitor/mouse on the back if this is more convenient for the system administrator.
In pictures: Taking apart Hewlett-Packard's Proliant DL380 G5 server