Play games
Choose the game that interests you and play itfor free.
Complete the quests
Every game has some quests for which you will receive Soul Gems.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
Game owners pays gamehag to advertise their games and the tasks is meant to make us addicted to the game. That way the game owners will recieve more payments, gamehag will get money from game owners and we get soul gems from some of the money gamehag gets. :). I hope this clerifies your questions!