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These straps were introduced with the "next generation" (Recaro) seats. They prevent the door trim panel from intruding into the passenger space during a side impact collision and are required with at least the next generation seats.
The door trim panel R&I procedures in the Service Manual have this warning: "Warning: Failure to install a retention strap could result in injury or death in case of air bag deployment."
Long story short, don't mess with it. If at all possible, keep it intact while you do whatever you're doing with the door. If you have to remove the strap, the Service Manual has a warning to install a new one and not to reuse the original one.