Decades ago manufacturers in many industries realized that they needed to test their products not just inspect them. Putting a toy or gadget into the hands of a potential customer would allow company researchers to see how it would actually be used and record the reactions of the customers both before and after their product experience. These types of Usability Tests have been adapted to the modern age in the form of website and software testing.
Software and website developers learned early on that extensive testing is the key to a successful product launch. The engineers themselves were much too invested in the software to be able to understand how the average user will interact with it. Likewise, the web developer sees a website through different eyes than the regular consumer.
Usability tests will make sure not only that software and sites function as designed, but that the user will be able to understand, navigate, and use the product easily and in a manner that is stress-free and enjoyable. If a site or program is too complicated or frustrating, they are less likely to become return visitors or users. A positive or even great user experience is what gets customers to not only return, but to recommend the application or page to their friends and family.