The vented door give extra ventilation for high heat output applications.
There is a large cut out on the rear panel that can be used to feed wires directly from the wall into the cabinet. Additionally, there is a cut out on the fixed portion of the frame that allows cables to be passed directly into the cabinet that have been routed on the outside of a wall.
The rails are adjustable so you can move them back in case you have to have more room between your equipment and the door. This is particularly useful when the rack contains patch panels, patch cords or cable management devices, ever wired rack and then found that the door won't close?
There is also a cut out in the top of the rack where you can mount a fan to increase the amount of air flow for powered equipment such as network switches.