idk they are just a waste of time. learning the content actually in the syllabus sets you up for trials + hsc + in-class tests. hand-in assignments are often a distraction and people spend just as much time on them as studying for the hsc itself.
ive seen it happen where people just ignore the content they are learning in class, and spend two weeks perfecting an assignment that they do decent on, but then it doesnt even matter cause the flop trials and hsc cause they are behind on content.
yeah its good to try hard on hand-in assignment, but dont let it distract you from learning content in class, even if its not related to the assignment.
hand-in assignments r really important tho, depends on the subject but usually very helpful + its big weighting. eg, in software our major project was 40% weighting, allowed u to make really good and detailed documentation, and allowed u to implement a bunch of different algorithms. was really good revision for all the documentation primarily, got a refresher on all the diagrams as well as ergonomic stuff so it was like free year 11 revision + algorithm practice but also weighted 40%, i can't see why u wouldn't want to try on that.
similarly in ext1, our research project was on bezier curves and physics problems, both of which i'd say gave me a better appreciation and understanding of vectors that i probably wouldn't have gotten if i just did textbook questions. bezier curves are slightly out of the syllabus but as a mx2 student something similar could have been feasibly asked, and physics problems can also be asked in hsc for ext1 and ext2, so it's good practice.
ur right that if u just do well on hand-in's and don't do any exam practice, then naturally u will flop the hsc and trials. but realistically hand-ins are also a good way to refine your knowledge and are usually only ongoing for a few weeks, after which everyone will just go back to past papers anyway. so yeah those few weeks u won't be past paper spamming, but it doesn't mean u are doing nothing inside the syllabus, adding detail to stuff that u would usually do in a few minutes under exam conditions can help u to understand what ur doing at a deeper level, even in english working on an essay for two weeks lets u really see where ur issues are and can let u improve in exams (although i never had a hand in assignment in english).