I disagreee, no pro has built good chest muscles using only flies and flies are never their main exercise it's there last and lowest on priority list., they only do a few sets at the end because you can't use much wheight on it and with heavy weights it also becomes dangerous to perform.
It's a good isolation exercise but it's not even close to beating the dumbell bench press because of the wheight ratio which is needed for muscle growth
Flies target the pectoralis major, because the pectoralis major is an adductor of the humerous.
Bench press is overrated for targeting the pecs, but they build good overall upper body strength because it involves a few muscles.
The dumbbell press is really good as well, but flies are the best because it targets the pecs specifically.
However, you also need very strong forearm. So the weak link in increasing the weight, is your forearm. As long as its strong enough, you can do quit a lot to target the pecs for muscle hypertrophy.
Also, for advanced athletes or bodybuilders just doing one exercise is never enough because sooner or later the muscle will adapt and will plateau, therefore you need a multiple exercises in order to progress.