Quests are tasks that you can complete for additional rewards. To see your activity and completed Quests, open the tab on the right of the screen and select the top Town Center icon. Here you'll see a list of your active quests, be able to collect rewards for your completed quests, access your Treasure Room to collect previously earned Quest rewards, and view your Battle Log.
For getting an edge in comparison to other player's, it is recommended to complete at least the daily tasks. There will be 3 types of quests and every quest rewards Experience points. And the Epic and Legendary quests, which are randomly generated, give players extra diamonds and in-game resources also.