The idea is that the teacher doesn't have to keep their eyes on a laptop screen and can mostly move about the classroom in the normal way that they would teach a lesson.
With the LED they'd only need to go look at the screen when there's a question. Actually, if you put the laptop in the middle of the front row, set the font size large, and used a clicker to go through them, the teacher could probably read the questions from anywhere at the front of the class. It'd still be an issue if you wanted to move about the back of the class while giving a lesson; but that's a hassle anyway, since the students' desks face the front, so IME it's rarely done.