It's very important thing to have. It basically means you can have 3 different legendary affix power (orange text), without actually wearing the item. You can use materials to turn a yellow item into a random legendary/set item. It's great for farming a specific item, if you use yellow gloves and upgrade to legendary, it will either be legendary or set gloves.
It's even better for farming a specific weapon for a legendary you need. It's how I got my Wand of Woh for my speed farming setup (I have literally never had a Wand of Woh drop from a mob for me, so without Kunai's Cube, I would still be crawling through rifts).
sounds like something i should look in to then im after a decent 1h weapson as still stuck with a 2h one which is slow!