Oh dear. Classical orchestras need them because we can't be expected to memorise all the notes, dynamics, tempo changes etc etc that we've rehearsed. In general, orchestral music is MUCH less repetitive than what most of us play and much more difficult to learn. In general, I learn the difficult bits, and carry on reading the easy bits, even in performance. The conductor? What's he for? Imagine how hard it would be for 80 musicians to deal with all the balance, tempo and expression details (MUCH more detail than we put into our performances) without some cues and help. It's a different world, so much more detailed.