When Nielsen issues their public rankings, it's a free sample. Okay?
It gets people talking. Some people enjoy lists. It's a competition, and you can see how stations rank in a very simple field. It serves the purpose, and it's perfect for listeners and other non-professionals. It builds the brand, gets the company name in the papers, and it might even get some more advanced people interested in the rest of the story, which is available for a price.
But it's not what professionals use for ad buys and hard analysis.