It's simple, the main determinant as to whether or not your website is going to be a success is usability and not just utility. If a user finds your website hard to navigate, they're simply not going to stay and more importantly they aren't going to go to another site. Let's face it, web users are impatient.
Though carefully crafted content is extremely important, the user isn't going to sit and read all of it, they are going to scan it and look for something click-able that attracts the eye. For a website to be effective, the designer needs to be able to communicate their message in a clear and concise way. Also, the less work the user needs to do the longer they are likely to stay.