Answer:
Size matters. Research has shown that people consistently eat more food when offered larger portions. So portion control is important when you're trying to lose weight and keep it off. A portion is the amount of food you put on your plate, while a serving is an exact amount of food.
Portion control is important because it allows you to have a tight handle on how many calories you are presumably consuming. This way, you eat what your body needs, instead of mindlessly overindulging.