Iron Chestlets you upgrade your storage space as you get further into the game.Instead of just crafting new ones, you can trade resources to expand your chests, so your storage actually keeps up with your progress.
Right now, the limit is the Double Gold Chest with 63 slots. This is due to some technical limits in the game's engine, but I'm already working on a way to get past that and add the rest of the tiers. I’m also building some extra add-ons and working on other mods too. Stay tuned for more updates soon!