Just a guess as I've never seen them before but if you've had OEM GM weatherstrip, you know that the ends of the rubber has a piece of think hard plastic molded into the rubber where the screws go so as to prevent ripping of the rubber.
On aftermarket, there is no plastic molded into the rubber ends just floppy rubber and a hole for the requisite screw. I've seen the rubber rip in these spots and suspect that these plastic pieces may be way to attempt to bolster the rubber to it will be less likely to rip.