Kurt Harris MD asks that very question in his excellent blog, PaNu, where he makes a convincing argument for ditching grains from the diet. He says:
I have never had anyone able to tell me exactly what evil would befall a person without wheat, barley or rye in their diet. I have scores of non-celiacs that say that it made a huge change for the better, and some say it did much more than the sugar elimination.
For anybody with celiac disease, and he maintains that 97% of people with CD are undiagnosed, the consequences are far more life threatening than most people realize:
Nearly every common autoimmune disease described is associated with at least an order of magnitude increased risk of celiac disease. Conversely, celiac patients have increased cancer, osteoporosis, and autoimmune diseases like DM I, autoimmune thyroid disease, Multiple Sclerosis, Sjogren disease, Rheumatoid arthritis, neuropathies, and even neurological disorders like schizophrenia. We don't know how big the iceberg is with these diseases, but the tip seems very large.
But he also makes the argument that in an evolutionary sense, meat is better for us than plant food, which is often adapted to avoid being eaten. And that makes the argument against cereal grains (notably wheat, barley and rye) relevant to the rest of us who do not suffer from celiac disease.
Not having legs to run away or claws and teeth to defend themselves, plants have developed passive defense mechanisms instead. They might have poisons or hard shells to make them less attractive to creatures trying to eat them.
If this is the first time you've come across a theory like this, you're likely thinking it's total nonsense. But please read his reasoning. Harris started off thinking it was nonsense too, but he's a smart guy. Read the article here. If you like it, there's also a second part, which you can find linked at the end. If anything, I find the second part even more convincing. Read them both and then make up your mind.