The usual reason for the IRD to make this request is because your travel dates may be wrong. Wrong travel dates mean that your student loan debt may be wrong too. When the IRD processes your IR886 this can lead to three possible outcome: the debt stays the same, it reduces, or it increases.
You may ask why you would want to fill in a form that could lead to your debt increasing. There is no point in trying to pay off, or negotiate a debt, that is falsely low. The IRD can come after you for the actual debt years later (plus interest and penalties) even if you thought you had paid it off. It is far better to know your situation accurately now so that you know exactly what it is that you are dealing with and can factor reality into your decision making.
You may be worried that by filling out an IR886 the IRD will have information about you. Not providing that information does not really protect you as the IRD has enormous powers to get your contact details and financial information anyway. By refusing to fill out an IR886 you are denying yourself the opportunity to make an informed decision, as well as aggravating your relationship with the IRD and this is something that can count against you if you are seeking their help.