I feel like that's the benefit to the creator having made this a dog (even if a demon-possessed dog). As intelligent as a dog can be, can we ever truly understand why they do the things they do?
The Uncanny Valley effect would be too big if a human companion randomly started talking to you while you were sneaking up on an enemy. But a dog? You'd be pissed off at it for a bit, and then forgive it because it's a dog.