You have to see it from the other side. It's about sustainability of the diet.
Eating 1800kcal of white bread, salad and skinless chicken breast will leave you with a raging hunger. Sure, you have a caloric deficit but how sustainable is that going to be? On the other hand eating 1800kcal of salmon, bacon, vegetables and eggs is the same caloric deficit but (for most of us) with way less hunger involved.
Plus your salmon, bacon, vegetables and eggs diet will provide micronutrients needed for proper functioning of organs and immune system. A micronutrient deficit might not be obvious on the short run but can do serious harm to the body long term.
Eating 1800kcal of white bread, salad and skinless chicken breast will leave you with a raging hunger. Sure, you have a caloric deficit but how sustainable is that going to be? On the other hand eating 1800kcal of salmon, bacon, vegetables and eggs is the same caloric deficit but (for most of us) with way less hunger involved.