If you don't understand what you read, you lose track of what is going on and your attention is likely to drift. This happens to some people sooner than others . Once you have lost the thread of the reading, it is awfully hard to pick it up again. Individuals with poor comprehension have a very difficult time grasping the whole of what is being read, although they may well get parts of the lesson or story.
Some people only have trouble when they read.
Others have trouble when they read and when they listen. It improves the ability to communicate as well as organize ideas for writing.