Of the three macronutrients, protein is the one people most consistently under-eat, and the one that matters most for body composition, recovery, and appetite control.
It's the only macro you can't make more efficient
Carbs and fat can be stored and mobilized fairly flexibly. Protein is different: every day your body breaks down and rebuilds tissue, muscle, skin, hair, immune cells, and it needs a steady supply of amino acids to do that repair work. Fall short consistently and the body starts pulling from your own muscle to cover the gap.
It protects muscle during fat loss
This is the part most crash diets get wrong. When you're in a calorie deficit without enough protein, a meaningful share of the weight you lose is muscle, not fat. Higher protein intake during a deficit preserves lean mass, which keeps your metabolism higher and keeps you from ending the diet "skinny fat."
It's the most filling macronutrient
Protein has the strongest effect on satiety of the three macros. Meals built around a solid protein source keep you fuller for longer and make it easier to stay in a reasonable calorie range without white-knuckling hunger all day. That alone makes it one of the highest-leverage changes in a fat-loss plan.
How much is "enough"
A practical target for anyone training regularly is roughly 1.6 to 2.2 grams of protein per kilogram of bodyweight per day, spread across 3-4 meals. You don't need exotic sources: eggs, dairy, poultry, fish, legumes, and a good whey or plant protein powder all work. Consistency across the week matters far more than hitting an exact number every single day.