If you try an interface out on four people, you'll usually find three of them hitting the same problems. Win Friends and Change Minds Usability testing finds problems with your product that you never could have guessed were there. For instance, you'll sometimes discover that wording that is natural to you gives entirely the wrong impression to a regular user. Other times, you'll find that buttons and menus that seem obvious to you are ignored or overlooked by users who either don't notice them or don't interpret them as being important to solving the problem at hand. It's these little things that add up to a sense of anxiety and confusion on the part of the user.
Usability testing is also useful from a political level within an organization.
For one, it's the evidence that matters when trying to settle an argument between different design approaches. Engineers of good conscience can have hellacious battles arguing which interface design is the best for solving a given problem. Although designs can take hundreds of hours to code, a few hours of usability testing can often settle the question. Another thing to remember is that usability testing can be wonderful public relations for your project. By taking the time to go out and recruit target users to help in your usability testing and product design, you'll have gone a long way toward spreading goodwill and the sense that you really care about your customers' needs.