What is an ad?

So I have been revisiting the thought of finishing my book finally (one of my new year resolutions). I mapped out the next few chapters yesterday and one of the thoughts is on advertising and what the purpose of it really is. I think advertising is an extension of the customer experience, which is fairly obvious, but for advertising to be effective there has to be a call to action. Purely branded ads have a call to action as well -- they want the user to go to the site and learn more or to investigate their products and company more.

Sometimes I think ad agencies have placed themselves in the position to create art in an advertisement. Ads aren't art -- they serve a function of communicating the business's values and experience. I think the other problem is that most companies don't know what their values are or what their experiences should be. Maybe both are to blame for insufficient advertising?