User testing is a way to test the functionality of your site on different platforms as well as the overall usability of the site to the end user. User testing can help uncover problems or ways that you can improve the end user experience.
User testing allows you to see how your site is being used and how the end user interacts with it. When a site is designed by a web designer or company, they have an in-depth knowledge of the content of the site and about the dynamics of the company. User testing can allow someone with "fresh eyes" to view the site and interact with it how a new visitor would. Many things can come to light by using this method like broken links, missing navigation options, or unclear information.
User testing can be done by having somone who is new to the site use the site. They can either be observed, or interact with the site and fill out a survey after. Heat maps are often used to capture clicks as the user navigates throughout the site. Often the user is given a task such as opting into an email list using a call to action, or finding information about a specific product. The process they use to do these actions can show how easy it is for a person new to the site to find this information as well as what problems block them from easily finding information.
While it's helpful to have the average person use the site, it can also be helpful to hire someone who does usability testing for a living as they may have a keen eye for issues that might be missed by developers. Developers and those associated with the company can also do careful walkthoughs. Developers can also do testing on various browsers, screen resolutions, and devices to make sure that the site loads as anticipated. Different browsers can render coding differently at times. And screen size can drastically change the layout and usability of a site if not coded properly.