When you want to get in and out of the gym quickly, it helps to know which exercises give you the most bang for your muscle growth. Squats are key for your butt, curls build your guns and for adding thickness, power and definition to your chest, the absolute king is the bench press. But, don't dismiss the fly completely -- it too adds a lot of value in your workouts.
The bench press and the fly obviously differ in their execution, but also in their focus. Each targets the pectoralis major, the primary chest muscle, but the bench press also benefits a number of neighboring muscles that the fly neglects.
You're best off including both in your chest workouts. However, if you can choose just one, it should be the bench press.
The bench press is known as a compound exercise because it requires work from multiple different joints -- namely the shoulder and elbow. Because of this, you see direct muscle-building benefit for the shoulders and triceps, as well as the chest. The more muscles you include in one movement, the more efficient your workout.
The fly is an isolation exercise because it targets one joint: the shoulder. While your chest muscles rely on help from the front of the shoulders and the biceps during the fly, it's really a chest-only exercise.
The bench press is best at activating the pectoralis major when compared to nine other common chest exercises, found a study sponsored by the American Council on Exercise in 2012. Incline dumbbell flyes managed to engage the pecs only 69 percent as much as the bench press.
The fly and the press focus on separate actions of the chest. This is why both are valuable when it comes to training overall chest and shoulder function. The bench press trains your muscles' ability to push weight; the fly trains your ability to adduct, or bring the shoulders and chest towards the center of the body.
It's up to you to determine which action is more important for you to hone. If you're after size, then the press is going to help you get bigger sooner. But, if you want a balanced, functioning chest wall, you can't leave flyes out of your workouts. Even if size is your primary goal, working the pecs from multiple angles helps you build more overall chest muscle.
When including both exercises in a routine, realize that the order in which you perform them will make a difference. For a workout that provides a maximum benefit from the bench press, it should be the first exercise you do, showed a study published in a 2014 issue of the Journal of Human Kinetics. Bodybuilders know this and often use the fly as a "finishing" exercise at the end of their chest routine, so they can work on lifting the most weight possible and being freshest for the bench.