OPC minister Chad Van Dixhoorn has written an essay on the the making of the Westminster Larger Catechism. He explains:
there seem to be two main reasons why [the Larger Catechism] was written: (1) creedal unity and, (2) more fulsome instruction in the Christian faith; as the Scottish commissioners envisioned it, the chief beneficiaries of the Larger Catechism would be the adult Christians in both kingdoms who understood the doctrines and duties of the Shorter Catechism already, and needed the meat of the Word.