An Instrument Rating is required for flight operations in adverse weather, known as Instrument Meteorological Conditions (IMC). Without this license a pilot is restricted to flight in good weather (known as Visual Flight Rules or ‘VFR’) conditions.
Operations such as offshore flying or Emergency Medical Services may require a pilot to hold an Instrument rating.
Nowadays most insurance will require an instrument rating when seeking a position as a commercial pilot.
An Instrument rating will train you to fly the aircraft by reference to the flight instruments alone, when there are limited visual cues outside the aircraft. You will also learn to fly in controlled airspace, dealing closely with air traffic control and making precise approaches in the busy Southern California skies.