I think that answers your question to "how is this a framework" and "what is the value in this?"
For many startups who want to get a site up quickly and have it look good and work well on a large number of sites, using something like bootstrap as the basis for their site design is a huge win.
In your previous posts you keep mentioning "designers", plural. What if started your own firm and didn't have money for "designers", plural, but wanted a site that looked good? Code all that CSS bugger yourself and test it across all those browser versions and different display sizes? Or would you maybe look for a solid, easily customized solution that is built around design and coding best practices and that is tested by an active community?
Do you code your own OS kernel and device drivers as well?
What's a good value for the ratio between your base font size and the lineheight you use? Hmm? A decent approximation is already used in the LESS code for bootstrap.
How do you implement a clean grid system that works across browsers and resolutions? How many grid implementations have you tested? Implemented? How reliable are they? How well do they work?
Are you hand coding your CSS? Bootstrap is probably what helped most of the people on HN realize the value of CSS preprocessors such as LESS.
Instead of spending a load of time optimizing common UI experiences across lots of browsers, or hiring a consulting designer who will probably reuse the code he's already written and charge you a chunk of change, you can instead use bootstrap. And when there is a point release you will find that they often included lots of little optimizations that make the overall CSS work better and across more browsers.
Frameworks provide templates and take care of the boilerplate for you. Bootstrap isn't any different from any other framework in that regard.