You can ask Jo a trillion questions about a site, but if Jo can't find something or use the site, then you know where to stop and make repairs before you move on. This is why feedback obtained though user testing is valuable in measuring the success of a site. This is particularly true of sites that sell something. You become frustrated and swear off the site for all eternity. User testing can prevent that from happening at your site.
Users provide pure and natural responses to a Web site.
They can tell you things you wouldn' t otherwise learn. They have opinions and information you need. Although sometimes their responses are startling, most often their feedback is helpful. Better still, user testing pays for itself. You may not want to add user testing to the cost of developing a site, but the feedback is valuable enough to warrant paying for a full-blown usability study.