Because we didn't always have access to multicolor (this was back in the day where we wouldn't put multicolor cards in sets without a multicolor theme), we decided the ability made the most sense in black
So think of this is your head: You get out one copy of this using One for One in Def mode, and you end your turn
However, MTG Arena has given players a way to be able to compete without the need to have a complete card pool at the beginning and that is through the use of the pre-built beginner decks that they offer every season to new players who start out in the game
One symbol means one Energy is needed while two means that two Energies are needed and so on
And, make no mistake, filling the 'yard accounts for half if not more of the power of this card, especially when you can build around milling yourself with your pick of Delve, reanimation effects, Delirium, Undergrowth, Karador, Ghost ChieftainKarador, Ghost Chieftain, Muldrotha, the GravetideMuldrotha, the Gravetide shenanigans