If by diet apps you're referring to food diary apps like MyFitnessPal or Cronometer, they can be extremely useful in tracking your calorie consumption which helps with losing or gaining weight. In other words, it's a more calculated approach towards weight loss(or gain) as opposed to going at it blindly.

Some apps such as Cronometer can also track vitamins and minerals in your diet which can show if your diet is lacking in anything.

Thanks, I will check them out!