The cellulose aka fibre is useful for them too, for the same reason as us, even if there are not many calories or other nutrients available. Dry food, bones, high protein diets can all benefit from a bit of bulk.

I've been told that wild dogs and wolves eat the digestive contents of herbivores which is semi-digested (free range haggis!), which definitely seems true to me given how much both of mine go for rabbit/sheep droppings. This supposedly supplies some more available vegetative nutrients to them. But this came from a pet nutritionist, I'm not sure how strict or scientific their training is.