There is a big difference between losing weight and losing fat.
When you lose body weight, by going on some sort of diet, your body will lose not only body fat but it will lose muscle as well, which is not a good thing as having more lean muscle on your body is very beneficial, especially as we get older, because we lose lean muscle from our body naturally.
Also when just focusing on losing weight, rather than losing body fat, most people restrict foods out of their daily nutrition, which then makes them feel deprived and very unhappy.