Why do you not think it's important for humans to eat the way they evolved to eat over 200,000 years?
You wouldn't try to feed a lion the same way you feed a Giraffe or Gorilla, yet you think you can eat any damn thing you want... WTF?

Well it is important that a humans modern diet should reflect what they ate during the earlier stages of their evolution (animal flesh, vegetation etc).
However ultimately everything we eat is broken down to the molecular level - glucose, fatty acids and amino acids. Our bodies can not recognise the source of the nutrients we take in - a glucose molecule obtained through the digestion of broccoli is chemically identical to a glucose molecule in a can of pepsi. Obviously the body benefits greatly from the micronutrients (vitamins, minerals etc) from "natural" foods that are not found in modern processed food items.
The intake of energy is what our physiology causes us to seek out above all else - modern junk foods are therefore craved because of their high caloric density.
Cats and all felids are somewhat different in that they are obligate carnivores. This means that unlike dogs, bears, humans etc. they lack the ability to digest plant matter and can not synthesise certain essential amino acids from food sources that are not whole sources of protein (i.e. animal flesh). In other words the best diet for your cat is 100% meat based. They supplement this themselves by ingesting certain plant matter like grass on occasion, simply to help with digestion or as an emetic. (In the wild, the skin and fur of prey would serve this purpose).