If there are no constraints on summoning minions, then this game would be almost entirely luck based. Whoever drew the strongest card first and played it would almost certainly win. Doesn’t sound fun to me. Let’s fix that by constraining the ability to play cards by their mana cost.
Now that we can play cards, let’s add a reason to do so. Our player’s deck is currently filled with nothing but minions, so you might be able to guess what this lesson is going to be all about. After playing a card that happens to be a minion, we will “summon” the minion. Instead of going straight to the graveyard, it will go onto the player’s battlefield, ready for combat.