Mostly for style but it does has a function. They are called vortex generators, you can find them on airplanes as well.
Ill draw us a pic
w/o vortex generator
with vortex generator
the red ares shows high pressure air, the blue shows low pressure air, and the orange shows approximate air stream over vehicle
sorry for the poor quality, but im doing what i can with what i got
especially note the differences in the airflow (orange) over the rear window, and its proximity to the rear window, which can be characterized by the sixe of the low pressure bubble (flow seperation) on the back window
so what does this show. if you concentrate on the VG section you will notice a difference. one (the one whith the huge VG) lets more orange in closer to the car's back window.
this means 2 things.
1. there is more airflow to the rear wing (if you have one)
2. the low pressure bubble behind the rear window is reduced.
low pressure bubbles (or seperated flow). in short, they are bad for aerodynamics.
think of this: how does a vacuum cleaner work? well it sucks air into it. how does it do that? by pumping air out thereby creating a low pressure, and the sucking is the air rushing in to equalize the pressures (its really that simple )
so effectively low pressure bubbles "suck" cars back (this is one of the reasons for rear diffusers and vortex generators, among other things)
how the vg reduces low pressure bubbles. the vg is exactly what it is named for. it creates vorticies. if you dont know a vortex is basically a "whirlpool" you might see in a draining bathub/sink, or a river. but this vortex is made in the air (because air is a fluid, just like water is a fluid) the vortex, if you notice, has a big side, and a small side. at the tip of the vg is the small side. as distance from the vg increases, the size increases. what this does, is infringes on the low pressure bubble, and "mixes" higher pressure air, with the top layer of the low pressure bubble, effectively moving the pressure gradient closer to the car than a non-VGed evo
so what the VG does, it reduces drag on the vehicle [by reducing the low pressure on the rear window] and it increases downforce [by allowing more air to pass over the rear wing's airfoil]
however the vg's also do create their own drag, because the frontal area technically would create a high pressure region, but the benefit outweighs the cost in this case
here is mitsu's official technical write-up on the subject (and they have real CFD pictures!!!!)
http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf