Subaru introduced the roof vane on their production STI in 2006. The link above gives a really good explanation of the job that it does. Basically, it directs air to the trunk spoiler and gives the car more traction and grip at high speeds.
However, I guess some other FIA World Rally Championship teams thought that this was an unfair advantage so the rules were changed and the roof vane is not allowed on the 2007 WRC race cars....
The rule changes don't apply to the Group N cars because those cars are based on production vehicles. This is a quote from P-WRC driver Toshi Arai:
"Downforce at the rear has dramatically improved with the group N machine since the roof vane was installed. Group N uses production cars, so I don't expect the regulation will apply to this category in the future either. If we didn't have the roof vane, we'd have to re-shape the front bumper and reassess a huge number of aspects on the car including suspension settings and machine height."