Whenever I've thought about the concept of zombie appetite, I've always had two questions: what do they gain from eating and what happens to what they've eaten? I've never seen or read anything that tells me the answers to these seemingly important questions. The only exceptions I can think of is Return of the Living Dead where a zombie explains that they eat to get rid of the pain of being dead. Okay, I'll buy that. In Dawn of the Dreadfuls by Steve Hockensmith, it is explained that consumed meat passes through the zombie's digestive system unprocessed and eventually is forced out of the anus as "zombie droppings".
That is what I had come up with before I read the book. It didn't make sense to me that there would be any nutrition absorbed by the zombie's body. In the Nocent books, swallowed meat just gets lodged in the back of the throat. Since zombies don't have to breathe, they don't choke or cough. The food just stays in the throat until more food is ingested. I just hadn't thought it all the way through to the excretion part like in the book I read. That book is the prequel to Pride and Prejudice and Zombies by Seth Grahame-Smith.
As to their appetite, my zombies merely eat for the purpose of the kill. Remember that they are possessed by the spirits of the Nocent. The Nocent have an agenda. They want to eradicate their enemies the Collective. Many humans are helping the Collective so the zombies are out to kill those humans. The Collective become vampires so that they can protect the humans. There are also humans that are helping the Nocent. Unfortunately, for them, the zombies will kill them too.