This is especially useful since a secularcourt would not be permitted to decide a contract dispute, for example, between two parties who had explicitly agreed to adjudicate their disputes according to Jewish law or Catholic canon.
It would be an unconstitutional "entanglement" problem (violating the Establishment Clause of the First Amendment) if a secularcourt had to get into the business of deciding what Jewish law or Catholic canon was and what it said about a given dispute.