This BOX contains reprints of cards from the old boxes, ones that are commonly used by many players and can fit in to many decks, as well as containing some new cards that will be released in future BOXes or Structure Decks. Each pack costs 200 gems to purchase and contains 12 cards, with each pack guaranteed to have at least 1 SR or UR, along with 7 N and 4 R cards. Gem purchase of the packs is limited to 15 per person, and will not replenish even if you choose to reset the BOX.
Although the animation only shows 5 cards being pulled from each Pack, you can view all the cards you received from the obtained card list.
You can purchase more packs if you decide to do the In-App Purchases, and there is no limit to how many packs you can buy.
I wouldn't say this box was necessarily important. Admittedly, it was somewhat useful for newer players that hadn't gotten any of the first few sets, but unless you're building Dark Worlds, you didn't miss anything vital exclusive-wise.
Double Cyclone is good, I'll grant you that. But Cosmic Cyclone is better, so missing it isn't quite as big a deal. As for Electro... I'd say pulling it normally is easier, actually, since it's from a mini-box.
Also, keep in mind they did say that the cards here would be released again somehow in the future, so it's not like anyone that lacks Double Cyclone, Dealings, etc. will never get them.