Answer:
Advantages: Their main advantage is their V/STOL (Vertical and/or Short Take-Off and Landing) capability which allows the Harrier to fly from short rough-field strips near to the front-line to provide rapid close air support (CAS) and hence the ability to operate away from conventional paved runways and airfields which may be attacked preventing conventional non V/STOL aircraft from operating.
Other advantages of it's V/STOL capabilities are the ability when in flight to vector the engine nozzles to perform movements without changes in orientation or profile of the aircraft which can confuse an opponent during ACM (air combat manoeuvring). This also allows the pilot to reverse the engines causing a rapid deceleration causing a pursuing aircraft to overshoot the Harrier, giving the Harrier pilot the chance to take the initiative during ACM.
It's relatively low speed may be useful during CAS as it allows more time for the pilot to correctly identify and engage ground targets.
Disadvantages: Highly skilled pilots are required to pilot the Harrier. There is a high maintenance requirement. They are relatively slow by fast jet standards and have a short range and payload.