Whether the purpose of your site is to convince people to do something, to buy something, or simply to inform, testing only whether they can find information or complete transactions is a missed opportunity: Is the content appropriate for the audience? Can they read and understand what you’ve written?
Consider a health information site with two sets of fact sheets: A simplified version for the lay audience and a technical version for physicians. During testing, a physician participant reading the technical version stopped to say, “Look. I have five minutes in between patients to get the gist of this information. I’m not conducting research on the topic, I just want to learn enough to talk to my patients about it. If I can’t figure it out quickly, I can’t use it.” We’d made some incorrect assumptions about each audience’s needs and we would have missed this important revelation had we not tested the content.