From Brotato Wiki
Piggy Bank is an in-game item that can be bought from Shop or can be found in a Crate. Most Items increase the Stats of the player while some of them provide unique effects.
Piggy Bank is a unique item. Unique items can only be bought or found once per run.
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.
Unlike the Materials gained by Harvesting, Piggy Bank doesn't give Experience. It only gives you the Currency Part of Materials.
While playing in Endless Mode, Piggy Bank stops functioning after wave 20.