It saves parliament time. For instance, around 3000 statuatory instruments are made each year, and so usually Parliament only has enough time to pass all government bills introduced, and not enough for all the new local laws required. Delegated Legislation can be passed quickly to deal with situations as they arise, like in emergencies, where parliamentry prcedure wouldn't suffice as it would take too long for the law to actually be passed.
An example act for this is the Cableway Insallation Regulations 2004. This act required detailed technical knowledge of cable cars, drag lifts and ski lifts to comply with a Eurpean Directive, and so it was made by the transport minister. This is a type of a statuatory instrument.
A further advantage is parliamentry control, which means that although parliament is not making these laws, they still comply with the requirements of the parent act, and so the will of parliament. There are also parliamentry control over delegated legislation in that statuatory instruments are scrutinised by the scrutiny committee,by laws have to be approved by the relevant minister and judges can declare void and delegated legislation that has gone beyond its power.
Lastly, delegated legislation is, to an extent, democratic, in that the government ministers, responsible for issuing statuatory instruments and approving by laws, are elected, and orders in council are drafted by the government, whom are elected, although they have to be approved by the queen and Privy Council, whom are not elected.