Maybe I can attempt to shed some light here. I suspect that it was placed there originally in an attempt to provide an amount of downforce on the ground, to increase tyre contact patch area and aid traction on slippery surfaces while rallying and also to counter the upward momentum of the rear mass of the car during jumps.
The designers at the time didn't have the benefit of the shared knowledge that we now have, so wouldn't have known that a wing (or spoiler, just an inverted wing, acting in the opposite direction) approaches it's critical angle of attack typically in the range of 8 to 20 degrees relative to the oncoming airflow. Consequently, I would estimate that the Stratos rear spoiler, as it is angled at way more than 20 degrees to the oncoming airflow, actually spends most of it's time fully stalled and creates more profile and induced drag than useful downforce, but there will be some downforce, nevertheless.
But hey, it looks cool, and as has already been mentioned, provides a useful area for advertising sponsorship!
All the best to everyone for Christmas!
Mark
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