Using HTML by itself has its limitations. I myself had wondered how it was possible to make things such as dropdown menus and icons for links to social media; the answer for that was implementing a UI, which in this case was Semantic UI. Implementing a UI into a webpage vastly increases the amount of options you can apply to the page.
It was a struggle to learn about Semantic UI on top of all of the new content I needed to learn with basic HTML. However I think that the struggle will payoff in the future. It gives web developers additional options to make their webpages unique. Imagine if all of the websites on the internet used the exact same format of HTML. How boring does that sound? The same concept applies to software engineering, UIs present a variety of options to expand the look and feel of ones software.