Despite their complexity, UI frameworks can be very useful. They allow for far improved polishing. Frameworks can be faster to learn compared to learning to bypass them. Using a framework can make a page much more usable.
Polishing - Using a UI Framework allows for much more aesthetic refinement. It is easy to achieve a high degree of polish. In contrast, refining a page without a UI Framework would be difficult to impossible.
Learning - UI Frameworks are difficult to learn and use. Using a Framework allows for a much more appealing appearance than HTML and CSS on their own. Developing good looking pages without a UI Framework is much harder.
Useability - In addition, a UI Framework can make your page more user friendly. An aesthetically refined website is more intuitive for the user. This is especially true for users who are not familiar with computer software.