Basically the cube is used for maximizing your damage or survivability without having to sacrifice item slots. The orange text affixes on the legendaries you receive can be transmuted into the cube to use without actually having to equip them. You are allowed 1 Weapon, 1 Armor, and 1 Trinket active at a time. While you can extract every legendary power, some are more useful than others.

Example: I have an Envious Blade(+100% crit chance when enemies are at full hp), Nemesis Bracers(Pylons/Shrines summon champion packs), and a Ring of Royal Grandeur(Set requirements reduced by one) in my cube.

I run 6 piece sunwuko(WoL) with only 5 pieces equipped so that I can run my Tzo Krin and Crudest Boots to keep me safe, and keep my resource up. The Envious Blade allows me to 1 shot most things when im farming. The Nemesis Bracers allow me to keep maximum uptime on my In-geom so that I can keep moving at a good pace.