Putting a decayed counter on a creature has two main functions it makes the creature unable to block, which is going to swing combat in your favor, and it also means that if its controller ever does decide to attack with it then that is the last thing that it will ever do (they have to sacrifice it)
A 4/4 with flying whose opening move is to destroy one of your opponents creatures is great, and its +1/+1 counter ability is just another boost to your bloodsucking army
It also happens to be a fairly decent card for The Weather Painters, which means itll see a lot of play amongst fans
The synergies are obvious, too
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Of course, the payoff for resolving this in Goblin decks is massive, as when it enters, you get to nab all of the Goblins with mana value five or less from the top six cards of your library and plop them directly into play