Your Piggy Bank (Pig Icon in a Chest) is one of the best deals in the game! It serves as a storage of the gem rewards you're earning.
Every level you beat puts more gems in your piggy bank.
As soon as you earn 400 gems, your piggy bank will be unlocked and you may collect your gems by making an in-app purchase at a huge discount.
Hope this hleps and let us know if you have other questions.