While playing as Arch-Villain Rafaam in Battlegrounds, his hero power is I’ll Take That!: Next combat, add a plain copy of the first minion you kill to your hand.
As you get later into the game where board space is a premium, you'd feel less inclined to play the cards from your hand. Or the ones you get don't have any synergy with your current build. But you want to hold onto them for the triple upgrades and the free tier upgrade that they provide.
Strategy aside of play the cards onto the empty space and selling for the coin; How many cards can you have in your hand?