The point being that some people just don’t get enough protein. As with sufficient calories, adequate protein is critical for gains in muscle mass. The common number that is thrown out is 1 g/lb body mass and this is a good starting place. As I detail in The Protein Book, raising protein to 1.5 g/lb (another common value) may have small, cumulative benefits that current research can’t turn up. It usually can’t hurt unless it prevents sufficient intake of the other nutrients.
I would note that, for natural lifters, I don’t see much point to intakes over 1.5 g/lb. An exception is hardcore diets but I’m talking about muscle gain here. As caloric intake goes up, protein requirements go down and suggestions to eat 2 g/lb for naturals seems more of a ploy to sell protein powder than anything physiological.
As a final comment on protein intake, it’s very common to find wannabe bodybuilders taking protein intake to the other extreme, and making it the entirety of their daily diet. This ultimately sort of ties into the first problem I talked about: inadequate calorie intake. For the kinds of caloric intakes that many people need to gain muscle/weight at any decent rate, it’s nearly impossible to consume enough protein to do it. It’s also inefficient as hell, both metabolically and financially but those are separate issues.
For example, a 170 lb male may have a maintenance caloric requirement of around 2500 calories/day. To gain weight, he may need three thousand or more calories per day. Three thousand plus calories or more from protein alone is nearly impossible to achieve.
This is on top of the fact that protein calories aren’t used as efficiently for energy as calories from carbohydrates or fats (this can be great for weight control but is a real detriment for weight/muscle gain). That’s on top of the fact that protein plus carbohydrates is far more anabolic than protein or carbohydrates by themselves. Studies have shown that, once protein requirements are met, more muscle is gained by adding dietary energy (from carbs or fat) than from just plugging in more protein.
Is sufficient protein crucial for muscle mass gains? Yes.
Is it all a lifter should be eating? Absolutely not.