Your Diablo III characters lead lives in which every new artifact is judged on its ability to crush, exsanguinate, and electrocute every enemy within a five-yard radius. So being told that Kanai’s Cube is powerful implies that it has power solely within the realm of cutscenes and your characters would just as soon throw it in the closest Horadic Trash Receptacle. But even though you can’t use the cube to kill things, you can use it to enhance other items which can kill things. It enhances them in notable ways, even!
For example, Kanai’s Cube allows players to store up legendary powers and apply them to new items, or even reforge items to have more desirable stats. It also allows players to transmute crafting materials and remove the level requirement from items. It also has plenty of lore elements and strong resonance for long-time players of the franchise, but let’s face it, you were probably sold by the whole legendary power storage bit.