When we try to lose weight we usually go on a low caloric diet and incorporate lots of exercise. After some time of sticking to a diet perfectly and workout nonstop, you will notice that your scale has gone down a few pounds. The problem here is most diets are set up for weight loss, so you will be losing some muscle mass along with your fat mass. This imbalance can set you up for more struggles later on and makes it even harder to keep the weight off.
One reason why it is important to keep muscle is muscle burns more calories at rest than fat does. That means, the more muscle we have on our body, the more calories we burn on a regular basis. Our muscle is one way to improve our metabolism which is vital in order to lose more weight or maintain the weight loss you just accomplished. This is another reason why most diets will lead to a plateau. You will notice weight loss at first, but since you are losing muscle, you are now slowing down your metabolism where you cannot lose any more weight.
Your body fat percentage is a much more valuable number than the number on the scale and should be taken into consideration for anyone who wants to lose weight.