When we are working on a big project, adding styles to all of our content can be a lot of work, especially when we are new to programming and are not not familiar with the tips and tricks to get our work done quickly. So, there are CSS frameworks that help us add styles to our content in short time without having to style each of our individual element. When we use a CSS framework like Bootstrap, we don't need to write as much CSS code as we would have to if we are not using the framework. In Bootstrap, all of the CSS is pre-written and we just call out a particular style for our content by simply adding the specific class in our HTML markup which then picks up that particular style from Bootstrap style sheet. This is a much more time efficient process of adding styles to our markup. Other than this, adding navigational bars and laying out our content on the webpage with the grid system is also relatively simple and it helps us making our webpage responsive as well.
Note: The Bootstrap styles is added before our own style sheet if we need to write some extra CSS so that our CSS sheet can over-write the pre-defined style sheet of Bootstrap.