Relevance is an easy principle to overlook for some businesses, but it is absolutely essential. Relevance is easy to see with information-based businesses. For example, if Google was unable to present relevant search results, searchers would quickly look for a search engine that was able to. But relevance is no less essential for other types of businesses.
Let’s say that you take your car to an auto mechanic and explain that it won’t start. If after checking the car out thoroughly he gives you a long list of things that are wrong with your car, most of which have nothing to do with why it won’t start, you will instantly realize that he doesn’t understand the essential nature of relevancy, and he's likely to have lost some trust as a result. Conversely, if you had told the mechanic that you needed a new starter, and after investigation he reports that the starter is fine but your battery connection is loose and corroded, this guy totally understands the nature of relevance. And having established a high degree of trust through this experience, he can provide you with a list of other things you might wish to do to your car, and you are much more likely to authorize those repairs.
Oh, and if he REALLY wanted to score an ACE, he could clean up and tighten the battery connections at NO CHARGE... then he's got a potential customer for life!