This is one of the most relevant exercises for your inner chest area. It will give you the much-coveted superhero-look to your chest - or should we say chest plates! It's not a movement that is designed to pack on mass - it's an exercise that will give you a nice line right in the middle of your chest and will separate and outline your pecs very nicely.
Thus, keep your focus on executing the movement properly rather then going really heavy with the weight. It's a finesse movement and such movements rely on proper pace and execution to be effective.
This exercise is one that you should use when you're on a muscle definition or toning cycle. If your goal is building muscle mass onto your chest, you should do dumbbell presses instead of flys.