This is a pretty common calculation for people who track protien intake in the US, since most people weigh themselves in pounds, and most nutritional information is provided in grams.

On US nutritional labels fats, carbs, and proteins are listed in grams, but we generally measure everything in pounds.

When calculating your macro nutrition for weight gain the general recommendation comes out to about 1 gram of protein per pound of body weight. This is just a coincidence, but makes finding out how much protein you need to gain muscle mass pretty easy if you know how much you weigh and given that all of our labels have that info in grams.

For the same reason they still use pounds for anything.

Because my gram always makes me gain the pounds with the food she cooks :)