Cats are carnivores, and they simply cannot live without meat and the highly bioavailable protein and amino acids (taurine, arginine, methionine, and cysteine and more), they contain, as well as certain fatty acids and vitamins found only in animal tissue.
Cats can develop serious health problems if they eat only a plant and grain-based diet, even when supplements are added to that diet. There are products out there for vegan and vegetarian cats, but if you look at the list of ingredients you will notice they contain non meat protein sources, nonanimal omega-3 sources, and a lot of vegetables and grains. This kind of food has very little nutritional value for cats. So sadly, those kind of diets can lead to major health problems, including dangerous deficiencies of key amino acids, fatty acids and vitamins.
Of course, it’s never a bad idea to add vegetables to a (raw meat) diet.