Why do cats eat fruit and veggies?

I hope you have triple checked to make sure that everything you have given your cat is safe for cats to eat.....one of my cats LOVES lettuce and if I have a salad I will give him one or two small pieces.....my other cat will put it in his mouth and then spit it out....lol! my vet says that a little bit of lettuce from time to time won't harm cats