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 Diamond Helmet
Enchanted Diamond Chestplate
Enchanted Diamond Leggings
Enchanted Diamond Boots
Enchanted Diamond Sword
Enchanted Diamond Pickaxe
Enchanted Diamond Shovel

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