Making a website is a lot like baking cookies. You have to have all the base content mixed together and settled before you start to shape the website into its final form. Once you have rolled out all the details, it is time to split all the content into pages. Not just any pages but beautiful pages that take up as much of the content as possible. One way to do this is to use a knife and carefully cut each page out with as much precision as possible. The easier way to ensure the pages looks like what you want is to simply use a cookie cutter. It not only speeds up the cutting process, but it helps use up all of the dough. After baking, it is time to style the cookies. A valid way to decorate the cookies, is using a toothpick to spread the frosting and then placing the sprinkles on one by one. While this can lead to some very nice cookies, the time it takes is painful. Why not use a knife to spread the frosting and have a shaker for the sprinkles while still having access to the more precise tools?
Trying to build a website using only plain HTML and CSS is like cutting the dough with dough with a butter knife and spreading the frosting with a toothpick. Eventually, you could get what you want, but it will take forever and the work will not be easy. Anything you want to accomplish can, in theory, be accomplished. However, in an industry where time is money, does anyone really have the time to do everything in its most basic form just for the same results? The answer is no.
In web development, Semantic UI is the Costco sized pack of cookie cutters. Every type of cookie cutter is there and can be manipulated to look as desired. The smaller tweaks to the Semantic UI components are like the frosting and sprinkles. They are not necessary, because the content is all there, but they add flavor and aesthetic appeal. By adding a UI framework like Semantic UI to the development process, you will not only speed up your work, but you see better results in less time. Simple class manipulations from a pre-existing library and a few easy CSS tweaks can result in professional looking websites with less than a couple hours of work. After working with cookie cutters, it is unlikely that I will ever go back to using a butter knife and toothpick.