What I meant was the the people who are dropping out after the SC will want to have a least a good mark to remember at least they did something good in their school years, and some people find year 11 hard so they drop out, and their SC mark will be their final school mark, so if they did shit in that its going to have a psychological impact on them and they will think that they really stuffed up their schooling life. SC is also important for some people because they might be changing schools etc And the people in selective schools will want a good mark as well as if say they get Ban 4's in all their subjects they is a chance that they wont be going to that school for year 11.
Edit: Some people actually want to try acheive something in their life and also that the SC will make the person actually study, because if you cant study for the SC how are you going to study for the HSC?