For someone serious about lifting/bulking, you should be eating 1.5-2 grams of protein per kilo of body weight, which can be difficult to get from dead animals alone. Even those who don't consume that much protein (I surely don't) may want to choose a protein shake to help balance their macronutrient ratios. Also, those who time their nutrient intake (particularly after a workout) probably couldn't eat 20g of protein within 45 minutes of working out.