Fair that hypertrophy is the most obvious answer and flys aren’t the first choice for it. But the truth is that “required” can be different depending on goals - mobility and peak contraction come to mind. A cable crossover fly, the only fly I personally recommend, provides great isolation (which you agree is important) and max elbow adduction that other exercises can’t provide. But if your sentiment is “there are better exercises” then I 100% agree. Incline dumbbell press is king these days.