Why do cats eat grass?

I am referring to the domestic cat. Although, it should be said, that wild cat species do eat vegetation as well such as the snow leopard which in particular likes to eat a certain bush! So why do domestic cats eat grass? There have been many theories for this and perhaps the most prevalent is that domestic cats want to make themselves sick, to sick up hairballs. Or, a slight alternative to this theory is that grass helps to push hairballs out of their digestive system so that they are defecated i.e. it is roughage. Another theory is that when cats are ill they want to make themselves sick so the grass is eaten to achieve this. And a further theory is that they like eating grass because it tastes nice.

The best theory is that the juice of grass contains folic acid, a vitamin which is not available to them within the flesh of animals. Folic acid is a naturally occurring B vitamin, specifically vitamin B-9. It is also called folate and is important in prenatal health in people. It is especially important to pregnant cats and dogs and it plays an important role in the production of haemoglobin. It is part of the cell multiplication process and helps to synthesise DNA. 

Domestic cats can also eat grass which is in the belly of prey animals such as rabbits. So cats eat vegetation or plant matter via their prey animals in any case. It is not something which is bizarre or out of the ordinary. What is out of the ordinary is that 50% of dry cat food is plant-based! How about that for a carnivore? People say that you should never feed a domestic cat a plant-based diet but we're doing it all the time anyway seems to me.