Well...
I'm just starting the HSC syllabus, but from what I have read it's basically the guidelines set by the developers on how the problem is going to be solved by them. So for example, with a scope statement, it tells them how far to take their problem by setting boundaries (as well as other things) so that later, they can do a cross-check to see if they've met the requirement specifications of the user.
So it can be seen that design specifications are also useful for measuring the success of a project.
