Well developed rear delts give the whole deltoid that desirable round look. As well, they fill out the upper back sitting comfortably around and by the teres, rhomboids, and upper traps. They give the physical appearance of a much more healthy, upright, athletic posture profile. And the dumbbell reverse fly is a standard for achieving that fullness.

Keep a rigid torso, shoulder blades slightly retracted and shoulders sitting high, not slumped.