People make notes for different reasons. Some say taking notes helps them to concentrate. If they didn't take notes they wouldn't be able to pay attention right through a lecture, or in the case of reading, they wouldnt really take in what they are reading Some take notes for later revision.
Notes can be an aide-memoire.
These are both reasonable approaches. But if concentration is your aim, it is difficult to pay close attention to a speaker and make comprehensive notes. While you are scribbling away you miss some of what is said. Taking notes from reading is extremely time-consuming unless you limit yourself to the main points. If notes for revision are your aim, you may think you will revise your notes later, but when it comes to the crunch the notes may not seem very meaningful and the exercise may prove to be so boring that you give up.