Many issues that come before the Supreme Court are not defined in law. Just to name one example off the top of my head... the issue of when is a person considered alive... is it at conception, viability, or birth? It's never been defined in law, but it is a key issue with regard to the legality of abortion. Another example that comes to mind is when the court had to decide whether increasing government revenue could be defined as serving public interest with regard to eminent domain cases. These and many more cases come before the Supreme Court so that they can interpret not only the constitutionality of the law, but also it's range and scope.
I think that's pretty much all you were missing.