It's my opinion that peas, lentils, etc. are added to increase protein without the addition of real meat.
Soybeans are often seen for the same reason. Many dog foods are made up of straight soy and little to no meat.
I am a bit...jaded perhaps, when it comes to dog foods. I feel those "high quality" dog food makers are cheaping out their foods, as well, and adding a lot of plant proteins in place of real meat - for instance, often the 1st ingredient will be "chicken" or "beef", with peas after that.
And thing is, after cooking it up, the protein from chicken or beef goes down because those muscle meats are mainly water until after cooking.
I still like Kirkland, 1st two ingredients are "Chicken, Chicken meal", and thus, more real meat content than a "higher quality" food with "chicken, lentils, peas, chicken meal" with chicken meal being after all those plant proteins.
That's how I see it anyway