Calcium is known best of all as a promoter of healthy and strong bones. But it plays a critical role in performance as well. Too little calcium and you can't relax your muscles. Bad relaxation, makes for less potential energy and so on and ultimately for less than satisfactory contraction. Bad contraction equals bad workouts and no growth. Get the picture? By improving relaxation you avoid cramping, injury and twitching.
Calcium is also very important in fat metabolization, so no cigar for the people who claim milk is fattening. It makes for less fat storage and better use of present macronutrients in the blood. Using the recommended FDA dosage has shown to decrease body-fat significantly in obese people.