The questions in the textbook are not as hard as the ones in HSC, mainly because they are only one topic at a time, whereas in HSC, in one question they can test you concepts from multiple topic. So that's what you can get from practicing past papers but not from doing questions from textbook
^That. It is important to have a "feel" of the levels of difficulty of questions in different past papers. It is also important to make sure you do those papers under examination conditions, mainly to calm the nerves (more practice = variety of questions covered = time management skills = more relaxed approach coming into the examination.)