We all know cats are carnivores. People who've switched their cats onto raw food diets likely did so because they know their cats are obligate carnivores. Their bodies are really only capable of processing animal protein; they don't have the enzymes necessary to process vegetable matter the way that humans do. And yet, many of us have also noticed that our cats like to eat grass, and perhaps other plants and veggies too. Why is that?
According to PetMD, there are several possibilities. The first is that cats can't digest grass, so it makes them vomit. When they vomit, they also eliminate other indigestible material from their GI tracts, which might make them feel better. In the wild, it might help to rid them of the feathers or fur from their prey. So it's possible that they instinctively go for grass when they're not feeling well to make themselves vomit.
It might also act as a laxative, helping them to pass difficult materials from their GI tracts, such as fur balls. Occasionally, instead of hacking up those fur balls from their stomachs, the fur gets into their bowels and gives them problems. A little bit of grass may help them to pass that fur.
Another possibility listed on PetMD is that the juice from grass contains folic acid, which is also abundant in their mothers' milk. Cats do need B-vitamins in their diets, so if they're lacking, they crave grass because their instincts tell them it has that missing nutrient. It's kind of like the reason we crave sweet things when we haven't been eating enough fruit.
But grass isn't the only plant that cats like to eat. Those of us who have plants around our house also know that our cats like to chew on them. Many plants are toxic to cats. However, some are okay. Love Meow says that broccoli is a vegetable that your cat can't digest, but it also isn't toxic to her. It's the same thing with alfalfa, cantaloupe, and even green beans. Just be aware that your cat might throw up fairly soon after eating these.
In fact, according to Pawnation, feeding your cat a tiny bit of broccoli periodically might keep her from eating your house plants. You still want to be sure your house plants aren't toxic to cats, though, because you never know when she's going to decide to take a bite out of one of your plants. If you want to know what's safe and what's not, visit the ASPCA's toxic plants database. You can filter your search results by animal (dogs, cats and horses), and whether it's toxic or non-toxic.
If you do decide to feed your cat non-toxic veggies, remember that she still needs the vast majority of her diet to come from animal protein. So veggies should be tiny, occasional treats only.