Coins can be used to buy Power Points for your Brawlers from the shop's daily deals. The ratio for buying Power Points is 2 Coins per 1 Power Point. Stars Powers can also be bought from the shop when the player has a Brawler who is level 9. Star Powers cost 2000 Coins each. In the form of a special daily offer, 500 Coins can be purchased once for 29 Gems.
The main use of Coins is to upgrade Brawlers. Once you have enough Power Points, you can spend some Coins to advance your Brawler to the next level, up to level 9. To get a Brawler to level 10, you do not need Coins. However, you must obtain the Brawler's Star Power.
Coins can be bought in the shop with Gems. Buying larger Coin packs is a better value per Gem.