The Horadric Cube is a needed quest item in Act 2 and 3, as you must put together a number of quest items into a new item in able to finish the acts. The Cube's functions go well beyond those two quests, though. You can use it over and over, and you will do so. It is useful for storage, as it takes up 2x2 spaces in your inventory, but opens up to 4x3. Most players use this extra space to carry back useful or expensive items to sell.
See all the Horadric [[Cube Recipes]].
=How to get it=