It is a well-known fact that most developers are not necessarily good designers. We prefer to work on the backend, building great performing and intelligent software, and sometimes, we tend to think of the user interface as an afterthought.
By using a predesigned HTML Bootstrap template, we can give our users an intuitive and well-designed user interface that was designed by a professional designer. If the design was based on Bootstrap, the developer is already familiar with most of the CSS class names, components, and plugins, and does not have to relearn anything.
The Web offers an assortment of free and premium Bootstrap templates. Themeforest (www.themeforest.net) provides a mind-boggling array of different premium site styles and designs.
With Bootstrap 4, the Bootstrap team also offers official themes, which you can purchase. Each theme provides a full set of tools and examples and can be used as a good starting point for your project.