Mulldrifter is there as situational card draw. It's the only card in the deck for dedicated card draw, and it's fairly easy to end up empty handed while answering everything your opponent does.
Mulldrifter can help refill, AND is available with a flying body for when you get to the grindy late game. You could substitute it fairly easily with some cantrips, or even a
Divination, but I like the flexibility.