Protein does three things no other macronutrient does as well. It's the most satiating — gram for gram it keeps you fuller than carbs or fat, so higher-protein diets tend to reduce calorie intake without conscious effort. It has the highest thermic effect, meaning roughly 20–30% of its calories are burned just digesting it, versus about 5–10% for carbs and 0–3% for fat. And it's the raw material for maintaining muscle — critical when you're losing weight, because weight lost as muscle rather than fat leaves you weaker and lowers your resting energy burn.
Practical target: research on body composition supports roughly 1.6 g per kg of bodyweight per day for people who train, with benefits plateauing beyond about 2.2 g/kg. Most sedentary people eat well under this. Spreading it across meals rather than one large dose is generally better for muscle protein synthesis, and there's no evidence that high protein damages kidneys in people with healthy kidney function.
Robert Morton and colleagues pooled 49 randomized controlled trials covering 1,863 participants to answer a question the field had argued about for decades: how much protein actually helps when you're resistance training? Their 2018 meta-analysis in the British Journal of Sports Medicine found that protein supplementation significantly increased strength and lean mass — but the benefit flattened out at about 1.6 g per kilogram of bodyweight per day, with a confidence interval extending to roughly 2.2 g/kg. Beyond that, more protein simply didn't add more muscle. The finding was useful in both directions: it validated that the standard dietary recommendation (0.8 g/kg) is far too low for people who train, while also puncturing the fitness-industry assumption that more is always better.