Short version.
They keep the cab from cracking at the rear window "V"s and also from possibly cracking or popping out the winshield in extreme power use situations like dragging!!
And also the areas just in front of the rear wheel fenders and lower rocker panels from rippling if I remember right..
I do know that my car lifted almost completely rigid after I installed my bolt-on Jegs S/F/C's in front of the rear wheel from the side........
pdq67
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