User Interface(UI) is the first thing that the user comes in contact with on your website. A good user interface attracts the user and helps to create a distinct identity of a website. This is why it becomes vital to test your UI, before pushing your website into production. UI Testing is a very important phase of development that opens up risks and possible breaks in the web application that could have been really hard to figure out.
Testing increases the quality of the web application and it would not be wrong to say that the reputation of the web application depends on how well it has been tested.