User testing is essential in building a website because it helps designers determine how people use it. Here are some of the critical roles that user testing plays in website design:

1. Identifying Usability Issues: User testing helps identify usability issues by observing how users interact with the website. This information is then used to make informed design decisions to improve the user experience.
2. Gathering User Feedback: User testing helps gather feedback from users, which can be used to inform design decisions. This feedback can include user preferences, pain points, and suggestions for improvement.
3. Enhancing User Experience: User testing helps improve the user experience by identifying areas of the website that need improvement. By addressing these issues, designers can create a more user-friendly and engaging experience.
4. Ensuring Accessibility: User testing can identify accessibility issues, such as problems with font size, contrast, or navigation. By identifying and addressing these issues, designers can create a website accessible to all users.
5. Increasing Conversion Rates: User testing can help increase conversion rates by identifying website areas preventing users from taking specific actions. This information can be used to improve the design and user experience, making it easier for users to complete desired actions.

In summary, user testing plays a crucial role in website design by identifying usability issues, gathering user feedback, enhancing the user experience, ensuring accessibility, and increasing conversion rates.