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Most of mine had one or two, usually where overhead cable, pipes etc might rub on the rail itself, that was/is my theory from waaaaay back, and never given anymore thought to them.
I remember seeing one concours vehicle and, as mentioned by Greg, the owner had attached all the tiewrap straps for wiring and pipes in line with those insulating straps. It wouldn't surprise me if that was their original purpose, to strop abrasion or damage to the rail at thise points.