I figure the bolts were tightened tight enough to keep the valance from moving. From what you can see when they're not covered, they're pretty beefy bolts with large heads and usually large area washers underneath them to keep the valance from pulling through so it would seem logical for the tape being used to smooth that out.
It was also common to put colorful tape on so you could see how close the valence was to the ground as the car went by. The lower you could get it, the more downforce the car would have (or so the commentators would say). I don't know if that was the reason the tape was put there or just a handy side effect of using it
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