Cutter will serve as an adviser to the president, helping to coordinate the Supreme Court nomination process -- a role similar to what Steve Schmidt played during the confirmations of Justices John Roberts and Samuel Alito. During those confirmations, Cutter helped head up Democratic messaging on the nominees.
Unclear yet if Cutter will be returning to Treasury after a nominee is confirmed, or who'll be stepping in at Treasury in the meantime.
P.S. Obviously, this isn't only about how large the SCOTUS fight looms relative to the economy. As Cillizza notes, Cutter has unique expertise when it comes to judicial issues, so it makes sense to detail her to the White House even if the economy's still very much at the front of administration minds.