This means there will now be a higher population of cards circulating in the market than ever before
Once you've built up your collection, you can build a variety of decks using 20 cards, then battle other players or challenge NPC trials
While bouncing three creatures isn't great for five mana, countering a spell isn't great for five mana, and drawing three cards is only okay at five mana, the fact that you can do things like counter a spell and draw two cards, bounce a creature and draw two cards, or bounce two creatures and counter a spell makes it so the sum of Mystic Confluence is much more powerful than its individual parts
If an opponent doesnt make a play worth interacting on, you instead can leap ahead with Memory Deluge or Thirst for Discovery on their end step, which easily digs you to your monster threats or situational blowouts like Blood on the Snow
Cards like Underground Sea are nonbasic lands with two basic land types, in this case, being both an Island and a Swamp