Coins can be spent on various things in the in-game store. Use /coinshop in-game to view the store. 

You can purchase anything ranging from:

• A lot of crate keys.
• /nick and RGB colours.
• /sethomes.
• Ability to use images on maps.
• Mob Catchers (Allows you to pick up and place mobs).
• Exclusive bundles.
• Other miscellaneous items.

These coins never expire and are permanent.