Gold coins are one of the currencies used throughout the game. Gold is received through many sources, and is the most easily accrued resource in the game.
The only action (which I can think of) not providing gold are the Chambers in the secret region.
The primary use for gold is purchasing cards from the altar, (5,000 for a single card, 22,500 for 5 cards). Read more about Coin Bag
The other, more noticeable use, of gold is card evolution / upgrading, which can be very expensive.