Hi!
In essence, sleeving is for the obssesive/compulsive.... Like me. I sleeved all of the wiring in my Rig. It is time consuming and requires patience. To get the correct diameter sleeving (the end result is a tight sleeve) requires removing and replacing most of the connectors.
Sleeving can aid in cooling if one is having thermal issues, it also makes the inside of the case look "clean" and "professional". It's the attention to detail aspect, purely subjective.
Got patience? obssesive/compulsive? Sleeve away!