If you play something like Elves, Goblins, Humans, or Merfolk, you can fill your deck with 30 creatures of the right type and never worry about your mana curve because these creatures exist at all reasonable points in the curve, but if you ever play 30 Giants in a deck, then the average converted mana cost will be five and you will lose most games without ever casting a spell
Plus, 1993 had some hilariously broken cards that either aren't legal or don't work the same anymore
Abilities that trigger when a player casts a spell (like Mirrorwing Dragon's ability itself) won't trigger
Or you can slot Zurzoth into the 99 of Nekusar, the Mindrazer or similar
Good-Fortune Unicorn, Renata, Called to the Hunt, and Master Chef are among the best counter distributors because they do the thing for free