Bootstrap is important, not just to myself but to developers worldwide because of the vast library it contains. No longer do you have to define and create your own classes. Classes are now imported as long as Bootstrap is linked and called on any element to style it.
CSS is not required in order to use Bootstrap styles. The styles come from the imported library. I wonder if you can combine the two though, it would only make sense. What if you like the style Bootstrap has, but have some CSS code of your own that you can implement?
With Bootstrap, scaffolding is much simpler, although I enjoy the CSS process of scaffolding. Buttons are a breeze with Bootstrap, alliteration anyone?
Bootstrap also facilitates the use of tables, typography, colors, pagination, drop-downs, forms, navigation bars and even carousel of images!