Everyone likes pretty and flashy things. If you have two websites with the same contents but one has better-looking layouts and menus then people would choose the one that is better-looking. This shows how important it is to make websites that are nice looking and user-friendly. But the using raw HTML and CSS to create modern websites would take years of practice and huge amounts of work. UI frameworks are the solution to that, it helps you create appealing modern websites under a short amount of time.
UI frameworks are beginner friendly tools that people can use to create professional websites. Using UI frameworks are much easier than using raw CSS if you want to create a good simply website. Because by using one of the UI frameworks, for example, Semantic UI, you just need to type in the class name and you will have a menu without actually writing the menu class and style it yourself. All the elements of a web page are made for you and you just need to combine them together into a real web page.
Another reason to learn and use UI frameworks is related to a very common yet important reason. We all know software developers don’t work alone, so in a team-environment sharing code and working together are very important. And by using a common and set UI frameworks will help everyone better understand what is happening. Imagin how hard it would be to look at someone else CSS file with class and ID name all over the place and try to figure out what you should change to edit something in the top menu.
UI frameworks provide simply elements as a starting point for beginners and expert can combine raw CSS with more advanced elements. Most UI frameworks are created for common uses, so they are more toward the general size. If you want something unique that is not inside the framework library, you will have to build it on your own. But you can then add it to your own library and reuse it later and maybe even share it with others.