I weigh just short of 15 stone, I have lean muscle mass with about 12% body fat that I wish to maintain by eating and drinking healthily and weight training. I weight train because it is more beneficial to my body type for remaining fit and healthy as opposed to cardio exercise which isn't really ideal for my body type, plus I also find it boring after 5 minutes.
Because of my training, I need to consume around 3k calories a day. That calorie intake needs to be spread over at least 5 small meals a day rather than just 3 larger meals, that gives the body time to process and use the calorie intake properly. Protein shakes consisting of milk, fruit and vegetables blended with protein powder is alot easier and quicker to consume, I have tried getting the extra protein intake from eating additional meals, but having the time to do so isn't always practical. So it is easier to have it in the form of a drink, plus whey protein is the easiest and quickest form of protein for the body to utilise during the day.
None of that makes me selfish or means I am consuming someone else's share. After all unless they have a similar fitness regime, they can easily drink water, or their chosen alcohol, or they can even drink one of the many milks made from nuts, oats etc. With their "normal" milk intake that would be relatively inexpensive, for me to drink that "fake" milk would cost me about £20 or more per week instead of about £5 that it costs me for the real stuff.