Using a UI framework to enhance the content of your website is a good idea to do so because just HTML and CSS will make a boring website with a cluttered stylesheet. When using UI frameworks like bootstrap it allows you to assign classes to the content of the website that do the same thing as the CSS with less clutter. As a result of that your HTML file will have a lot of tags and could be confusing to look at if you are not the one writing the code. I am all for readability and not having to ask the coder or someone else to tell me what a piece of code is doing. I like it when I can read someone else’s code and can easily know what it is doing and not have to think about it or ask someone else the same question. Even though it is less readable you do not have to put that much on the HTML file for a website to look good and presentable to the public.
Learning how to use a UI framework like bootstrap and master it gives you a lot of things in your tool box of making websites. Bootstrap has a lot of classes and features that are like CSS but do not require you to make and tinker with a CSS stylesheet. In the same amount of time as you take to make a website only using HTML and CSS you could make the same website but more presentable. You could do all the same thing that bootstrap does in purely CSS but that would take a lot of playing around with stylesheet and to make a pretty cluttered and messy stylesheet. Bootstrap makes it where it is easier to use and more efficient than purely using HTML and CSS to make websites. If you only purely use HTML and CSS to make a website the resulting website looks dated and boring compared to its bootstrap counterpart.
I believe that software engineers that use UI frameworks benefit a lot because they are more efficient and the ending product is better to look at. Software engineers that use bootstrap or other UI frameworks do not have to have two files that they have to modify and tinker when using HTML and CSS. They just have to mess with one HTML file that has a lot of the classes from the frameworks that modify what is being displayed for the finished product. Although you can do the same things in pure HTML and CSS it is a lot easier to do it using a UI framework like bootstrap. When using bootstrap there is more you can do with it that you could add when you are first writing the code and the first time seeing the product could be more like the finished product. The one downside I see is that to change the style of the website it would take a lot longer because of the classes you used, not just changing which stylesheet you are using.