> With no other identifying info, though, what can they do with a license plate number in isolation?

For typical users not taking extra precautions, visiting a page in a browser is providing additional identifying info, a fact that monetization of the free-as-in-beer web relies heavily upon, but which can be leveraged in other ways, e.g., by a site that draws you in with privacy fears as a technique to get you to submit additional information that can be correlated with it.