"But, why not hard wire under the hood to plug into??"
I had mine hard wired for a while, but the wing nut kept backing off and gave me all kinds of electrical gremlins. That added with the warnings about stripping/ruining the stud/wiring harness made me rethink the whole thing and go for the cig plug. Either way works fine, but since the older cars don’t have windows it is not much of a compromise.