From Brotato Wiki
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.
The Piggy Bank give you 20% of your Materials when you exit the shop and start a new wave.
For example, if you leave the shop with 100 materials, you will start the wave with 120.
While playing in Endless Mode, Piggy Bank stops functioning after wave 20.
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.
Piggy Bank is an Economic Item that can earn you a ton of Materials if you're ahead and can afford to save up Materials between rounds. Like other Economic items, it is more powerful when bought early, and of little value when bought in the late game.
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.
So when you have the Piggy Bank, you need to carefully evaluate how many materials you want to spend when you're shopping. It might not be worth it reroll as much. And some economic items might not be worth their cost, as the materials could be saved and multiply over multiple waves.
If you can't save up materials between rounds, it might be better to skip the Piggy Bank, since it will barely recoup its purchasing price.
The Piggy Bank has strong synergy with characters such as Demon or Streamer that naturally tend to have materials saved up between waves. The Character Saver starts the game with Piggy Bank and has their entire gameplay built around the item.