Mana Base Symbols represent the number of times each mana symbol is present on the casting cost of all the cards in this deck
transformed) side of the card
Where Im looking to include this is in decks that cant abuse the top of deck manipulation that much
While at MagicCon Vegas in late September this year, I spoke with Mike Turian, product architect for Magic (though his role while M21 was in design was principal product designer, according to his LinkedIn), and he confirmed that for M21 the design for showcase cards was intended to keep the text boxes clean and uncluttered, kind of like you would see in simple but powerful spells with only a few words on them, like Wrath of God or Counterspell
Pound for pound, one mana for a 2/1 creature with no drawbacks can't be beat
In the case of the card we have here, a variant of Emrakul, the World Anew, you can see from the stamp on the card that there are just 250 copies of this card printed