The most natural function of the upper chest is flexion and horizontal adduction, something the bench press can't do alone. Low to high cable flyes perfectly mimic the line of pull (and action) of the clavicular pectoralis. It's one of the best exercises around for "filling in" the upper chest up near the collarbone.
Low to High Cable Flye
Start with two pulleys set in the bottom position and have your palms facing forward. Your upper arms (humerus) should be at about a 30 degree angle away from your sides.
Using your upper chest to pull your arms up and in, raise the handles up and together so that they come together at shoulder level or slightly higher. The path of the cables will draw an upside-down V.
Here's Christian Thibaudeau coaching the exercise: