when revsing the aim is to remember all the vital information and be able to retrieve all the neccessary infromation in an exam (recalling information that you have just learnt) so just passively reading a text book or going through class notes won't neessarily help as you may understand the information you're givern but without practising and trying to actively recall this new infromation you wont be able to retain anything for the exam so doing diffrent things like taking things out or adding things in means you're actively using your brain to help you undertand and therefore more likely to remenber the information under pressure.
