Well, the shifting points kinda become almost a 6th sense to you
If you've driven the same car for a long time, you can have the music turned OFF and not looking at the speed, and still shift at the right points, because you're used to it already
however, when using automatic, you arehn't really gettin FULL CONTROL of the car
think about it, when a car changes a gear, the momentum is lost for a brief second, then it gets back
but, during that loss of momentum, the car's balance will be "destroyed" therefore, making it harder to control the car
however, if you use manual, the shift can be held to a later point.