Why would anyone possibly make this a law? Are they worried someone will see your parking lights traveling down the interstate on and assume you're in park?
A great deal of the vehicle code in the US is designed to give LE an excuse to detain. As you point out, there’s practically zero value to such a law, but, if it is in the criminal code, it’s a violation of law, allow police to stop, detain, cite, possibly search, seize and arrest. So, for an enterprising young officer who knows the vehicle code inside and out, these violations make easy pickings or “poaching” as it is sometimes called.