The Piggy Bank is a special offer in the game where you can buy Gold Bars for a much lower price than the normal Gold Bars packages you find in the game.
The game's concept, design, implementation, support and maintenance is a long and complicated process which needs to be monetised in some way.Since the game is already free to download and free to play it offers the Gold Bars as in-game purchases instead to monetise it. This includes also the Gold Bars in the Piggy Bank, but the Piggy Bank just gives you an opportunity to purchase them for a better price.
All levels can be completed without any purchases so you can continue playing without any Gold Bars. All purchases all fully optional in the game.
You have all details and info about the Piggy Bank in our games here.