The buff itself is not important; it flags you for a variety of phase change at the Altar of Quetz'lun. Until you complete the quest chain, when you approach the altar of Quetz'lun you will "leave your material body behind", moving into the 'underworld' (with the buff Ghostly). The world will appear in the monotones used when you are a ghost. This is similar to the effect of the Bash'ir Phasing Device.