Gold Chests are obtained as a reward from almost every reward source in the game. Since Gold is a resource you can only keep a limited amount of at a time chests function as a way to keep a much higher amount at any time than your current cap. This can be important because Gold prices are often quite high, especially for higher level building upgrades and can be quite intermittent, as higher level building upgrades can take weeks to months to complete. And making sure you keep tens of millions of Gold on hand at all times can be frustrating so make sure you always have at least a few Gold chests on hand at all times. This is especially important regarding Speed Scrolls, whereby you can complete building upgrades instantly, but still have to pay your entire bankroll for the next upgrade. Also there is an Event which rewards players for completing building upgrades so many players choose to save up as many Speed Scrolls as they can until the event and then spend them all at one time, this technique requires saving just as many Gold Chests to pull off.