Coins can be used to purchase more lives if you run out of them. When you run out of lives you will see a green circle with a plus sign in it. If you tap on this icon you will be able to purchase more lives.
Coins can purchase power-ups/boosters such as the chilling cloud, flower power, and duckling rain. When you run out of boosters/powerups you will see a green circle with a plus sign in it. If you tap on this icon you will be able to purchase more power-ups/boosters.
Coins can purchase extra birds if you run out of them while playing a level!