What is the credit notes tab on my billing page for?

It is empty for almost everybody, and that is the normal state. A credit note only appears if a charge is reversed after its invoice was already issued. It is a separate document with its own number rather than an edit, because an invoice that has been issued is never rewritten or deleted — that is what makes a numbered book worth anything. So if you ever see one, it is the document that formally reverses an invoice you already have, and your accountant needs both.