I think that UI frameworks help make it easier to make a useable and functional website. With a framework you can have a complicated looking website with the least amount of work. I use a framework at my workplace, and it is very easy for me to make changes to the site.
Making these sites with just HTML and CSS would take a lot of time and work. If you tried to make the site with just HTML and CSS, you would end up with a CSS file that was longer than your HTML code. Each small thing that you will need to make will take another CSS code section with the characteristics of your new section.
Using a UI framework is good for a software engineer. Using a framework will allow for ease of coding and for consistence throughout the code. It will also help if the coder is on a team or if they are coding for something that will last longer than their time doing the coding.