By tracking how much of each macronutrient you consume, you can impact many things such as body composition (fat vs muscle mass), energy level, and even your mood. No single macro is responsible for fat gain or loss (you gain fat by eating more calories than you burn).

Tracking macros rather than just calories has several benefits. Keeping protein high enough ensures that you will retain as much lean mass as possible during a dieting phase, and that you will adequately repair and build muscle in a gaining phase. Adequate levels of dietary fats are important for general health. Eating the right amount of carbs allows you to maximize efforts in the gym, and keeps you feeling energized. So while it is your calorie amount that ultimately determines whether you will gain, lose, or maintain your weight, tracking macros allows you to maximize your results.