If you mean the one that bolts to the C-pillar, on the roof, it keeps the airflow attached to the roof of the car so it doesn't interfere with the flow over the boot spoiler.
But, as Kym said, normal road speeds would barely create enough flow to do anything.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i