Supreme Court decisions are the next-best thing to black-letter law. It is important to clarify what the law has been decided to be, once and for all. Lower-court rulings have no shortage of legal jargon and terms of art with meanings that conflict with everyday meanings of those same words, because they're just part of the sausage making that is drafting and interpretation of the law. SCOTUS rulings can have these too, as the other responder indicated, but there's just fewer of them (and more clarifying language where necessary) because again, a SCOTUS ruling is a proclamation to the people, once and for all, of what the law has been settled to be.