Its ability looks pretty insignificant at first but combined with certain cards, it can help you produce infinite mana, buff your creatures with a gazillion counters and draw at the same time, or even have infinite turns
GR Gold Rare Graff, Malebranche of the Burning Abyss X DARK Level 3 [ FiendEffect ] ATK 1000 DEF 1500 If you control a monster that is not a "Burning Abyss" monster, destroy this card
Lower numbers are generally more exciting, but if you open the biggest number in a run (like a 500/500), thats usually worth more than a middling number, too
Some of them are as likely to put a blade in you as greet you, so it may be best to heed my advice
Entreat the Angels, Starnheim Unleashed, Finale of Glory, and Decree of Justice all reward you handsomely for spending a lot of mana on them
When its flipped up, you destroy the top 5 cards from your opponents deck