This big beast costs 8 mana in total and is a 5/5 with haste, but it also has a powerful ability: "When this creature enters, creatures you control gain trample and get +X/+X until end of turn, where X is the number of creatures you control." If a player has a board of around 10 creatures, and they slam down a Craterhoof, they're likely to take out multiple opponents in one turn
Add to this that the card has very good stats, 4/3 for 4, it has a boost effect by discarding and you have a creature very good at blocking and attacking
There should be more than enough reasons to play this card and its pretty easy to trigger, too
He watches the pass in both directions, and all must pay his toll
Then the appropriate player gets priority
By venturing through the Lost of Mine of Phandelver, you are able to venture to the Dark Pool room which drains the opponent for one life rinse and repeat until the opponent has no more life