APR Carbon Fiber Vortex Generator - Subaru WRX / STI 2015+
you are looking at the APR Performance Vortex Generator. These have been designed to channel airflow that flows over the roof to hit the to of your rear spoiler for improved downforce.
Here are more details:
NOTE: Part #CF-818014 - This will fit ALL 2015 - 2016 WRX / STI models and 2017+ WRX Base / Premium models with the standard 6.2" Starlink Radio. This will NOT fit models with the 7" Starlink radio.