Dieters have to have a reason to start a diet. Most people don't diet for the fun and enjoyment of it. They have an ultimate goal in mind. It could be to reach a goal weight,

How To Measure Weight Loss - by Jennifer Olsen
The scale has always been used to measure success of a diet. After all, weight loss is a common goal of most diets. However, it is important to clarify the difference between weight loss and fat loss. Since most dieters want to lose fat, not just weight, it is important to understand the difference.