It's a guardian exclusive item that allows you to hold more gold than the limit which I believe is 2 billion. You buy the misc and in the future you can resell for 1 billion. Players hoard these for future need and typically used for donation events.
The idea is that you buy it to store a lot of gold in your vault since the vault allows you to have multiple copies of the same item. For example, I currently have around 25 gold storage chests in my vault, meaning I effectively have 25 billion gold in store.