Statutory Instruments are not the same thing as an Order in Council: Statutory Instruments are 'delegated legislation' (the power is delegated by Parliament), whereas Orders in Council operate through the Royal Prerogative.
They are used because they are much faster and simpler to implement than a full Act of Parliament. SIs are sometimes described as "secondary legislation, not second class legislation". They have the same force as an Act of Parliament, and much of the U.K.'s law is made in this way. There are literally thousands of SIs each year, compared with a few dozen Acts.
They are also used to bring Acts of Parliament into force: it is not uncommon for quite major pieces of legislation to be passed by Parliament with all the clauses 'turned off', and a power for the Minister to 'turn them on' (or 'bring into force' as it is properly called) at a later date. Some sections are never brought into force at all.
Canada also uses the Statutory Instrument nomenclature. An example being the Proclamation of the Queen of Canada on April 17, 1982 bringing into force the Constitution Act, 1982 also known as the Canada Act 1982 (U.K.).