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Item
Piggy Bank is an in-game item that can be bought from a Shop or can be found in a Crate. Most Items modify the Stats of the player, and some of them provide additional effects.
Unique
Piggy Bank is a unique item. Only 1 of each unique item can be equipped, and it will then no longer be offered in the Shop, and no longer drop from loot crates.
Piggy Bank
Piggy Bank multiplies the character's Materials by 1.2 (adds 20%) at the beginning of a Wave.
Unlike the materials gained from Harvesting, Piggy Bank does not give Experience.
Piggy Bank stops functioning after wave 20, and becomes effectively useless. This is only relevant in Endless Mode.
The item tooltip shows the total amount of materials generated by the item, but does not take into account the purchase cost of the item itself, if any.
Strategy
Piggy Bank is a good economic item to buy on most characters and in most situations. The item will only become net positive if you can afford to hold back some materials. Very often this just happens passively, because it is rare to fully exhaust the material supply in most shops. Even with plenty of materials to spend, rerolling becomes uneconomic due to the increasing cost of rerolls per shop, leaving the character with unspent money.
The Piggy Bank is especially good on Demon, because this character pays with HP instead of materials, and will invariably have a lot of unspent money in his account.
The amount of materials generated by the Piggy Bank is substantial, due to the high growth rate of 20%, and because the process is multiplicative (compounding). Any amount held back will more than double over four waves, and will have multiplied by ~6.2 over ten waves.
Therefore, many of the other economic items in the game become bad investments with a Piggy Bank, because it is better to not buy the item in question and instead let the materials multiply in the Piggy Bank.
If you own a Piggy Bank, you need to carefully re-evaluate any economic items before buying them. Recycling items from crates for materials becomes a lot more valuable for the same reason (even economic items).