That's not an oem rear wing.. The side skirt has the turned rear section...most likely either an oem aerokit 2 or a knockoff.
And yes, Porsche designs their aerokits for aerodynamics and not simply for looks.
Here is an example of both aerokit 1 and 2. Both come with rear wings...kit 1 is very hard to find these days....
Aerokit 1 (scroll down the page) -
http://home.comcast.net/~kilsen/boxster_hacks.htm
Aerokit 2 -
http://www.****************************************************.com/product/000044802.html?Category_Code=986AK