Answer: The main reason is to do with the change in the rules over the winter, but it isn’t that these fins are now mandatory.
My article on the changes drivetribe.com/p/ERK3H-MQSe6LV2mEOjHsrg?iid=J99y9_JOSraEYDIEQF-kRA )
The main reason is the 2017 rules changed the position of the rear wing, making it lower. In this position the wing is no longer jacked up high above the car, the airflow reaching it is now obstructed by the roll hoop and the general airflow passing closely over the car’s bodywork. With this dirty air the wing is less effective and thus it needs to be set at a steeper angle of attack, to make the same downforce level. With this wing angle comes drag, along with the huge new tyres, this will really slow the car along the straights. So, the teams want to smooth the airflow heading towards the rear wing. As the rules restrict where you can put bodywork, you cannot have a horizontal vane\wing ahead of the main rear wing to clean up the airflow like they had in 2008. So, the only option is to use a shark fin.
Shark fins were popular a few years ago, firstly for their aid to airflow and then to be used as part of an F-Duct set up in 2010. When the FIA stepped in and banned F-ducts for the next year, the fin had to stop short of the rear wing.
They work in three ways;
Firstly, they act like wind-vane, the large side surface area doesn’t like to face the airflow, so should the car tail slide (yaw) the side force acting on the tail fin will try to correct the slide.
Secondly to straighten the airflow towards the rear wing. So again, with a tail slide, the airflow will arrive at the rear wing at an angle, being rectangular the wing prefers a head-on airflow, so the straightening effect allows the wing to work as it was designed.
Then thirdly and perhaps most importantly for 2017, the airflow tends to flow along the fin, rather than pass through an open gap where the flow can get untidy and turbulent. As the airflow leaves the end of the fin, it’s a much cleaner laminar flow, so the quality of the flow is much better and this allows the wing to be more efficienct at creating downforce with less drag. In the WEC (i.e. Le Mans) for the sports prototypes categories, the fins are mandatory. This is for slightly different reasons, the cars were found to want to flip over when the car slides sideways. To prevent this the fin exerts a leveling force to keep the car from flipping over, so their example is slightly different to F1, being safety rather than performance. Cynics could add the fin adds a lot of space for sponsor logos! Although many teams paint the fin over to make it less obvious to the TV cameras and trackside fans. The fin does have drawbacks, it’s a large and heavy panel, mounted high up that’s bad for Centre of Gravity. Also, the fin is sensitive to side winds, so very bad for the Mercedes launch\filming day, that was incredibly windy, so one wasn’t fitted for the day. Also, the fin includes its own drag from its large surface area, but this is overcome by the improvement to drag overall. Lastly the yaw benefits mentioned above are effective only up to certain angles, when the car slides too far around the fin actually blocks airflow the rear wing, losing rear downforce and potentially making the car even harder to control. So, teams are finding the benefits stack up to use a shark fin, but the rules do not demand that a fin is run for this season. In fact, the rules for shark fins have remained the same for many years and essentially unchanged since 2011 when F-Ducts were banned. We haven’t seen one fitted to the Williams 3D model nor to the Mercedes, but both are expected to test a fin and perhaps race with one. But these aren’t a fix-all solution, at various points the teams will test without a fin, the driver may prefer the feel without the fin and for some teams its likely they will drop the fin for the rest of the season or at certain races.