For experimenting and throwing an example together. So you have something in mind, maybe a wireframe or something one of your designers has made, or a customers existing site.
You go to bootstrap, hook into the CDN and start throwing some tags down. Boom. Bootstrap have you, you've invested time, effort and emotion into it. 90% of coders move on to the next profit making exercise. I know I did.... Sure, there are better CSS frameworks out there. But bootstrap is better than the crufty stuff we used to use, so it's win win win for me.
