They can amass a lot of 5/3 tokens quickly
His Eminence ability, which triggers even if hes in the Command Zone, means this 4/4 with First Strike and haste can create Vampire tokens as you cast spells of that type, and he empowers them with +1/+1 when he attacks
Its also a great enabler in Rakdos decks that love to dole out damage, particularly in an Aristocrats-style build
He arrives with plenty of loyalty counters, he can lock down your opponent for a few turns, and if left to scry and draw cards, you'll wind up with an emblem that will almost certainly win you the game
Pollywog Prodigy can rapidly get a handful of +1/+1s on it, and at that point youll be drawing a bunch of cards
I love most of those properties - yellow sponges aside - a lot, but I've never really understood why I'd want a deck of cards based on them