SEO starts with the intentions of the person using the search engine.
What ever the web searcher intended to gain from typing their search keyword/phrase into google will help you understand what needs to appear in their search results to entice them to click on your money generating link (a link that generates a sale, which was found using a transactional keyword).
The web searcher's intent is "why" they performed the search in the first place.
Ambiguities between why a web searcher searched and if a particular transactional keyword causes them to click the link when it is not related to their intent, results in wasted Pay-Per-Clicks; and this is why knowing the intention (the why) of the web searcher's search is important. (Am I confusing AdWords with SEO here?)
Additionally, the text displayed in an individual search result (the page title, and the slug text) can match the web searcher's intention and cause them to click the link resulting in a sale, which is what we as webmasters want.
So as webmasters, I ask you, what do you do to determine why people are searching for your transactional keywords?