Often a user can find himself on your site after a googling, without a particular reason that drives him toward it. At this harsh realty you have to add the statistic data (J. Nielson – Web Usability 2.0) for which averagely a user spends about 30 seconds on a site, after he generally leaves it returning to the page results. Summarizing, you have 30 seconds to convince your user that your site is exactly what he’s looking for, or at least he’s on the good way.