Certiorari is the instrument by which the Supreme Court has complete control over its docket. It grants review only for “compelling reasons”: a conflict between federal courts of appeals, an important unresolved federal question. A mere error by the lower courts is not enough.
The consequence is the one to remember, because it is constantly misreported: when the press writes that “the Supreme Court upheld” a decision by declining to review it, it is wrong. The Court upheld nothing. It considered that the case did not merit its time.
The reasoning is exactly the same as in France for the non-admission of an appeal to the Court of Cassation: the decision challenged becomes final, without any higher court having said that it was well founded.