It can't be overemphasized that in order for one person to be good in something, he must develop not just one but a set of skills. In basketball for example, to be a good player, one needs to be good at dribbling, shooting, passing, rebounding, agility, etc. Each of these individual skills had to be practiced over and over until a certain level of mastery is achieved.
The same thing with being a good student. One does not simply come to school, listen to the teacher, study and take tests to be a good student. There are certain foundational skills that should be practiced over and over. One of these skills is reading and, obviously, comprehension. One study by Sarah Sparks showed that students who can't read at their grade level are thirteen times less likley to graduate in high school. There are numerous other studies linking reading skills and comprehension to success in other subjects as unlikely as mathematics. After all, some mathematical problems are word problems and therefore require good reading and comprehension. Whole methods are even developed to improve reading/study/comprehension and SQ3R is one such method. The acronym actually stands for SURVEY-QUESTION-READ-RECITE-REVIEW. It is beyond the scope of this article to discuss this method in detail but suffice it to say that its development is a result of the need to develop the skill we are talking about.