The piggy bank is like a token keeper that allows you to save more tokens for the end of the Valley season.
In addition to regular tokens that go to the wallet, extra tokens will be rewarded when you complete building requests and they will be stored in your piggy bank. You can check the third tab of the Valley shops to see more details.
At the end of the Valley season, you can choose to open your piggy bank with 25 diamonds, and then those tokens will be added to your token wallet. The sum total can exceed the wallet limit, which will give you the chance to buy up to 2 items from the exclusive shops.
After you have bought all the affordable items in the Valley shops, the leftover tokens will not be cleared, instead they go directly to you piggy bank of next Valley season. If you choose not to open your piggy bank, the tokens you saved will also be transferred to the new piggy bank of next season.
Keep in mind that there is a maximum limit of piggy bank. Use it in a smart way!