Early on, Choco-Comet gives you a very relevant body to help you push damage
The three main rules are that each player must have a deck of at least 60 cards, each deck can contain only two Ink types (Amber, Amethyst, Emerald, Ruby, Sapphire, and Steel), and each deck can have no more than four copies of a card in it
Furthermore, the camp site they use is included as a new land that synergizes with birds
On top of that, anytime you smack an opponent with Koma you get to make four 3/3 Serpents, adding 12 power to your side of the board
There's no way you get a better creature for only 4 mana in the Oldschool format, which is also why it's heavily wanted
This can go both ways as you can either opt to mill your opponents or self-mill to create massive card advantage