Once upon a time Yellow pages had so much cash they did not know what to do with it. Not sure, the same could be said in 2017 where they have been dwarfed by Google but is Yellow pages dead?
Google is more than a search engine, it uses GoogleMap to its advantage when displaying results and has an incredibly friendly UI (User Interface) which makes searching seamlessly easy. However Google is not a business directory. It displays web pages based on how well the business's website mets certain criteria i.e. keywords, popularity, backlinks and many more whereas Yell just displays results from its record. Therefore even if a business does not have a sophisticated website and SEO (Search Engine Optimisation) the user will still see relevant results.
Example- Search for a plumber on Google and Yell. Google will only display plumbers who have their site optimised whereas Yell will simply present a list from its database. So if you are a customer searching for a business you will find more businesses on Yell.