Detect problems users have with your design and correct these before the product goes live.
It is impossible to get the design right in one go, no matter how much experience you have. Ask a small group of users to perform tasks with a (paper) prototype of your solution. Typically users are asked to think aloud so you can grasp what the user is thinking. In a formative user test the goal is to detect as many problems as possible users are encountering using the site. In a summative user test user behaviour is tested against pre-set goals.
Usability testing is common practice in many design firms, although sometimes placeholders are used rather than ‘real’ end-users.
This video (in dutch!) explains how Rabobank uses usability testing.