They add in vegetation to mirror a dogs natural instinct to forage for grasses, fruit, and vegetables. It is not a necessary part of any carnivorous diet unless the diet is sufficiently lacking in meat. If we are to use the argument that you want to feed a diet that mirrors what your dog would be getting out in the wild, well, if your dog lives in an area with junipor berries and mint leaves, they might just be eating that mixed with whatever other food they can get their jaws on. There are vegetables in there that I would argue your dog would probably never eat or come into contact with if they lived in an area with no access to humans, but that's beside the point as dogs are "canis lupus familiaris" - they're domesticated wolves.
Dogs are scavengers, they eat what will satiet their hunger. I think ACANA does a bang up job of offering a high quality kibble which mirrors (to a degree) what your dog might find if they were scavenging, however you're not feeding whole foods which is what your dog would be finding out on their own, so it is a very processed form of what their natural diet might look like.
