Schwartz wrote in 1966 for direct mail and magazines, and his underlying claim is that desire is not created: it already exists in people, and the job of an advertisement is to channel it towards a product. From that come his two contributions: the buyer's states of awareness and the market's stages of sophistication.
What I did with it
Awareness changed how I open a piece of writing. You do not write the same way to someone who already knows a solution like mine exists, to someone who only feels the problem, and to someone who does not yet know they have one. The first gets the product, the second gets the problem, the third gets what is happening to them. Writing one text for all three is writing for nobody.
Sophistication is the harder one to accept. When a market has heard every promise — and enterprise systems has heard every one of them — promising harder does not work: you have to explain the mechanism. That is why my pages describe how the work is done rather than how good the result is. In an exhausted market, the mechanism is the only thing still being read.
And a smaller rule I apply: the headline is the part doing the work; if it fails, nothing after it matters.
What I left out
Nearly all the rest, which is the machinery of another era: mailing lists, coupons, catalogues. And the cult around the book, which strikes me as the worst thing that happened to it: absurdly priced editions, people quoting it as revealed scripture. It is a dense, technical, well-reasoned text with two big ideas; treating it as an oracle is the surest way not to apply it.