The major pro is that it normalises the air flow over the rear wing.
If you imagine the airflow over a car driving around a fast corner you will see that it does not strike the rear wing at a perpendicular angle (because the car is turning). Since rear wings are designed for maximum aerodynamic efficiency when the car is travelling in a straight line, they do not generate as much grip around corners.
A shark fin will redirect air towards the rear wing at a perpendicular angle when the car is turning, thereby creating a situation more similar to when the car travels in a straight line and achieving higher levels of downforce.
In 2010 it’s also useful to have a shark fin to support the F-duct, since it happens to go exactly where you would like to release airflow.
A major con is increased vulnerability to side winds.
