For completing a series of tasks for the Irellians in the Level 1000 Eternal Plains realm you can gain special items that equip to your back. The main use for the back slot is to gain Wings and other such interesting items.
Known Back Slot Items
For a full list of the various items and what they do please read the Library > Items > Items screen and look at the Back slot.
All of the Wing “type” items have Air Survival on them, which may be required to access certain buildings in the realm.