It can give a creature islandwalk to sneak past an opponent with islands, shrink something by -1 toughness while adding +2 power to push through damage, or even force a player to discard a card

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You won't see much milling going on in white MTG decks, but black, green, and occasionally red see a mill card or two
New Card: Titan of Littjara Chief: I recently wrote an article about Sun Titan that included a few jokes about Frost Titan, so I was excited to see a new take on this cycle in blue
Sneaking through your small creatures is a great way to gain Max Speed, mind you
What they are not is a substitute for a proper 0 mana limitless sac outlet