[xyzzyb]

Bootstrap isn't a CMS. It does nothing to manage content but is instead a visual framework of HTML, CSS, and JS. It's essentially a good baseline of nice looking defaults.

It's popular because design is hard. Seriously hard. And the skills required to code up a cool web application do not often overlap with the skills required to make a site that isn't hideous to look at.

Bootstrap allows you to have a permanent check on the "design site" in your list of things to do for launch.

Not only does it look great out of the box, but it's also a great starting point for real designs (hence the name "bootstrap"). http://builtwithbootstrap.com/