States would still like to reserve that exercise for themselves. But the Court's pronouncements are a primary source for international lawyers, scholars and policy makers. Practically every paragraph of a decision or advisory opinion is scrutinized, probed, examined and quoted. Most cases and advisory opinions lead to publicists filling journals and books with hundreds of pages of analysis and comments. In that sense, the Court plays a significant role in saying what the law is for a great many people.
States make treaties.
States make the practice and manifest the opinio juris that build customary law. States, in doing these, look to the work of the Court. The body of work created by the Court is probably the most influential factor when a state acts in the context of making or applying international law. It is a truism in most legal systems that hard cases make bad law. In the international arena, so far, hard cases make no law.