wikidata= tag is for the feature itself.
operator:wikidata= tag is for the company, as the name implies.
A bunch of electrical wires is not a company... A company manages and operates them..

Another example let's say there's some notable building that has a wikidata article on it. Building's article would go into wikidata= tag and property owner or real estate company or management company would go into operator:wikidata=