One reason for this is that more complete markets reflect the relationship between borrowing costs and terms. Making an explicit yield curve reveals information about the expectations that the economic agents have, which is essential for business decisions.
Recent experience has shown that the development of bond markets must be based on placing funds in domestic currency, which is not only relevant for the transmission of the monetary policy but also as a mechanism to reduce external vulnerability. I would say that financial integration in general plays and important role enhancing the power of monetary policy instruments.