Orders in Council are used when an ordinary Statutory Instrument would be inappropriate, such as for transferring responsibilities between government departments. Orders in Council are issued "by and with the advice of Her Majesty's Privy Council". Orders in Council were used to transfer the powers from Ministers of the UK government to those of devolved assemblies.
Further information on this subject can be found from the following links.
Brief Guide: Delegated legislation ( PDF 350 KB)
Privy Council Office website