Many argue against this one which us understandable as the research/ studies into this is still unknown but it’s said that balance over time is created by variety!
From a calorific point of view, all proteins contain different levels of calories depending on the level of fats within, Lamb being a fattier meat than chicken.
So it's fair to say “Hypothetically” if you were following the Bodyweight % feeding method and are feeding 3% of your dog's Bodyweight (per day) in chicken, the chicken would contain far less calories than lamb, so if you fed predominantly lamb your dog/cat may gain weight on 3% but if you fed predominantly chicken your dog/cat could lose weight on 3% this is why variety can help with weight management if you’re feeding using the bodyweight % method as 3% from a 50/50 split of chicken and lamb could let your dog maintain their weight.
(At the end of the day it’s Calories in Vs Calories out, with various mitigating factors this was just a Hypothetical example of why variety is important)
Now lets go from a bioactive compound perspective, Bioactive compounds are elements of foods that contribute towards bodily function in a positive way, a couple examples which i'm sure you’ve come across especially if bodybuilding enthusiasts: Conjugated Linoleic Acid (CLA) which is an omega 6 (Helps with weight management), Taurine(Essential for heart health) and Creatine (Helps build muscle and speeds up recovery post exercise)
Every food item you can think of has bioactive compounds, the above listed are from meat sources only and every animal (Whole Prey) has different levels of these bioactive compounds, even within their own body which is why whole prey is superior for achieving balance!
This is why we’d recommend you feed a bare minimum of 5 animal proteins (1 being fish) and adding in some fresh fruits, vegetables , herbs and oils in rotation with the proteins, this helps tick these nutritional boxes and hit the aforementioned balance over time. However if you’re managing to feed allot more animal proteins than 5, i’d say 10+ (inclusive of fur/feathers) frequently rotated, then you may not need the plant based foods adding in, these are just a cost effective way of getting more variety in, There are some bioactive compounds in plants which dogs can utilise and plant based ingredients are cheaper than meats, so if money’s tight then rotate those fruits and vegetables in!
Remember dogs have zero nutritional requirement for plant based carbohydrates , but as dogs are facultative carnivores, they can get all they needs from a varied meat based diet but statistics show that most raw fed dogs these days are deficient in Iodine, Magnesium, Manganese, Vitamin E/D3 ,Omega 3 this is due to several factors but commonly because most follow an 80/10/10 ratio which has been shown to be lacking in the above so to help get around this you would need to rotate things in such as Tinned Sardines (Omega 3/Vitamin D3), rabbit/cow ears with the fur (Manganese), vegetables such as Kale (Magnesium), Mussels (Iodine), Cold Pressed Extra Virgin
Olive Oil (Vitamin E).
Plant based ingredients especially herbs pack a medicinal benefit to and are great to add in, so even as there’s no nutritional “Need” there most definitely still benefits to be gained by adding them in!