Much in the same way Ruby on Rails made every site work exactly the same. It's a framework to provide a foundational starting point that is already consistent rather than trying to build a consistent style guide and toolset from scratch every time.

One purpose is to allow less design inclined developers to rapidly produce a decent looking prototype that people can play around with.

Bootstrap is nothing but a foundation, and while many sites simply take one of the examples and run with it -- which is a perfectly fine approach if you simply want to put some information out there in an accessible format -- many others use it simply as a structural foundation. I've worked with CSS for years and am quite honestly tired of the BS (primarily the hilarious endless recreation of the layout of the archaic table element -- somehow it became verboten and we've been trying to re-make it with CSS for years) and simply starting with the grid layout of bootstrap is a great start.