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.
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.
Try saving up coins for brawlers that you like to play or want to push rather than spending it all on upgrades for other brawlers. This allows you to push easier and earn trophies faster during mid- or late- game.