FanYield is free-to-play

 Even if you don’t own all 6 players, you can use the loan feature to complete your lineup completely free of charge.  

So why buy player cards?

 It gives you more freedom. You won’t have to rely on loan players being available, and you’ll always have the certainty of building a lineup with the players you actually own. 

Another advantage is that player cards have limited rarity — so if demand grows, you can sell them at a higher price. 

… kind of like buying Apple stock back in 2011.