When you purchase this key, you get to choose which of the 7 items you want. This is not luck based.

In the server, you can choose one of these:
Enchanted Iron Helmet
Enchanted Iron Chestplate
Enchanted Iron Leggings
Enchanted Iron Boots
Enchanted Iron Sword
Enchanted Iron Pickaxe
Enchanted Iron Shovel

The armor can give you more protection which makes it easier to survive while the tools can help you mine blocks faster.