At the dawn of humanity, and for much of our history since, meals were literally catch-as-catch-can. Because humans evolved in a world where food was available only intermittently, survival required that we have the capacity to store ingested energy for times when none was around. Adipose tissue, familiarly known as fat, is the organ specialized for that task.
Our ability to store fat remains essential to life and can allow a person to survive starvation for several months. In very recent human history, however, the amount of energy packed away as fat has been increasing in many populations. When fat storage approaches a level that compromises a person’s health, we call it obesity.