It is slightly reflective and is there to help other drivers recognize that the sliding doors are open at night... this is in the event you've got the door open on the side of the road in the dark and not visible or inside the vehicle.
Personally, I've added a larger strip to all doors of a more reflective (safety yellow) material so that they're even more recognizable.
I do this on all of my cars so that they're more visible in the event that I am stopped on the side of the road with a door open.