why practice exams?

ok so my question is why do people recommend to do practice exams instead of reviewing content? for me majority of my subjects are content-heavy, so I thought it would be best if I just went over and revise all the content during the holidays before exams, but everyone says its a waste of time and you'd be better off doing practice exams instead. but what if you have forgotten all the content? is it still recommended to do practice exams ?? this is a bit long LOL sorry but its just been driving me crazy