Parliament doesn't have the time to consider and debate all laws, and if they used knee-jerk tactic of rushing bills it would pass many ineffective bills. Delegated legislation can be passed under a crisis too when Parliament are unable to handle it, e.g. Fuel Crisis 2000. Over 3000 statutory instruments are passed each year.
Parliament may not have the necessary technical expertise or knowledge required; for example health and safety regulations in different industries need expert knowledge while local parking regulations need…