Gluten is a difficult protein to digest. It is thought, for some people, the immune system sees the gluten particles as the an enemy and attacks it (producing antibodies) causing inflammation in various parts of the body and poor absorption of nutrients which leads to various health problems.
There is no evidence that everyone needs to avoid gluten. For many, it has become a fad diet or a way to seek weight loss, however that does not mean it isn’t a real condition. Gluten creates real health problems for many people.