I was reading the Wikipedia page for the U.S. Court of Appeals for the Second Circuit when I noticed that a "Circuit Justice" (in this case, Ruth Bader Ginsburg) was mentioned.
I then found the page concerning Circuit Justices on the Supreme Court Website. However, while this page lists the Circuit Justices, it does not explain their role.
I also searched the Wikipedia page for United States courts of appeals but did not find any mentions of Circuit Justices, except in the section on the history of the courts, which did not explain what Circuit Justices do in the present day.
My question is, as stated in the title: What are the roles of Circuit Justices?
I would also like to know how Circuit Justices are appointed and why they exist.