A person's diet is the building block for their health. Diet is the source of vital micro and macro nutrients for the body to function properly. If there is a lack of nutrients or an abundance of nutrients, then the body finds a way to over-compensate, or adjust to continue functioning as best it can. However, this will take a toll on the body, maybe not right away, but over time. The body's output is going to be based on the input it has received.
That is how a person with good health and no genetic health concerns will maintain good health. What if you have food allergies, diabetes, an auto-immune disease, a food intolerance, or one of the many genetic diseases that can affect health? Diet is still just as important, and in lots of cases can be more important, because the body will be more sensitive to the imbalances in most cases. Imbalances in micro and macro nutrients, in many cases, can lead to worsening conditions of already present health concerns.
This is why diet is at the core structure of society. Everyone has to eat and drink to stay alive. If you look back at every ancient society, diet is what dictated the type of society and the size of that society. For example, the nomadic tribes throughout the world would follow herds of animals that served as their main fuel source; planting and harvesting only what grew quickly and what was available in the natural landscape. The more stationary society is centered on a long-term agricultural and livestock development to supply the population with fuel. Both types of societies build culture, religion, beliefs, stories, traditions, and life around the way the world operates. At the heart is the lifeline of food or fuel for diet.
Today most people have access to food throughout most of the day. It doesn't matter if you're at the hardware store, grocery store, pharmacy, or traveling. There is always some form of food along the way. The problem is that we need far less food than we think we do, but since it’s there, why not eat it? This is where the over-abundance comes in, specifically an over-abundance of foods that are empty calories (foods with little nutrition value). Since humans have a natural drive to eat, it is hard to turn down all the food options all the time. This is when people run into health problems such as weight and weight related diseases.
Diet matters, because it is a function of everyday life that all living organisms share. We all need some sort of substance to fuel on; that is the natural drive. In modern everyday society, food is the core part of traditions, cultures, emotions, celebrations, and daily routine. Since diet is always there, people either don't pay attention to what they are eating, or they feel they have a grasp on their diet. Others are somewhere in-between. This is a simple function that has gotten very difficult over time. When you notice your diet is a struggle, I suggest going backwards and asking why diet matters.