Every effective diet is ultimately a reduced calorie diet. Some achieve this through macronutrient restriction without conscious caloric reduction and others by direct calorie counting. It is the latter sort of diet, one that focuses directly on calorie intake that I will focus upon here.
Portion reduction predates the concept of calories. It is plainly obvious that starvation leads to weight loss and so it is not surprising that people would conclude that deliberate weight loss should also involve starvation be it total or partial.
Indeed deliberate portion reduction is effective and total starvation very much so. Unfortunately, total starvation leads to death and therefore hardly constitutes a lifestyle change. Even used in the short term, total starvation is extremely dangerous and causes serious loss of lean body mass including muscle and vital organs. Total starvation plays no legitimate role in weight control today.