Card Delegation makes possible for people who don't want spend lots of money to play with maxed out cards. Now you can rent cards for cheap prices to play with them. Advantages of renting cards instead of buying is that before buying you can test cards to know which cards you'll play frequently with which cards are more useful etc. Renting cards will help you to reach higher leagues, get more cards from the daily quests, win tournament prizes. Also you can lease your cards to make passive income if you don't use them. Most expensive cards to rent are legendary summoners and some good Legendary Monsters.