While it is always important to design a website that looks pretty and makes search engines happy, it is always better to make sure users like your site. User experience is equivalent to customer satisfaction, but unfortunately some designers only test their usability at the end of the process, if at all.
Testing usability throughout every stage of the design process possible means the end design will be completely tailored to what your visitors want. Testing in different stages can tell you what you should focus on next and what can be improved overall.
Some people confuse usability testing with A/B testing, which is comparing two different versions of a web site on a wide audience and has quantitative results. Usability testing is all about subjective experience and qualitative insights.
As Jenny Shen puts it in her article at Onextrapixel, “Through A/B testing you can find out which version performs better, but usability testing helps you know the reason behind the results or why one version was preferred over the other.”
Shen also explains every step of usability testing in her article. I suggest reading it and taking notes so that you can start implementing testing on your next design. It won’t matter if you make a nice looking site if users don’t enjoy using it.