ONLY PROTEIN CAN PROTEIN
Often we hear (and I have said) when it comes to body composition change (fat loss versus muscle gain), it comes down to a matter of calorie balance (calories taken in versus calories spent). While this is true, there are some caveats. One of them is protein.
A gram of protein is equal to four calories and thereby contributes to the total calorie input. In this manner, it is the same as carbs and fat (which also obviously contribute to total calories for the day, four and nine respectively per gram). However, there is one major difference that separates protein from carbs and fats.
Protein is the only nutrient that is responsible for the repair of tissues in the body. For our focus, this of course is muscle tissue. On the other hand, carbs and fats can both be used for energy. This is why people can function in a high fat or a high carb diet. You can even use protein as energy. However, your body can't use fat and carbs to repair muscle tissue.
That being said, while yes, hitting total calories for the day is primary, if you don't meet your body's protein requirements, you are really limiting not just performance but also recovery (and a whole host of other factors). Carbs and fats cannot make up for missing the mark on protein. This is why I focus on a macros based approach to help specify calorie content.
So yes, hit your calories and also nail your macros but while there is a bit of leeway between fats and carbs, don't neglect that protein.