Because if it comes filled, this card essentially costs only 1 Mana, since you can immediately make use of the 5 Mana it granted.
Pretty insane in Aggro decks to have a 5/5 body on top of whatever 5 cost card they can play, while simultaneously shut down big board clears such as Flamestrike or Psychic Scream.
It will get rid of empty Mana Crystals first. If you play on 6, you'll have 0/5 Mana Crystals.
If you're at 10 Mana and you play Zihi, you'll be at 4/5 remaining.
Liv
It will get rid of empty Mana Crystals first. If you play on 6, you'll have 0/5 Mana Crystals.
If you're at 10 Mana and you play Zihi, you'll be at 4/5 remaining.