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 Netherite Helmet
Enchanted Netherite Chestplate
Enchanted Netherite Leggings
Enchanted Netherite Boots
Enchanted Netherite Sword
Enchanted Pickaxe
Enchanted Elytra

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