Bootstrap 3 is a responsive grid that uses media queries. I can trivially adjust the look of a site on desktop/tablet/mobile, all just by adding a few classes. When you add in the fact that they manage all the cross-browser/platform quirks for you, it's quite a winning proposition to use Bootstrap to start off a new site. You'll certainly end up overriding various colours, margins, etc. but that's the easiest part of CSS anyway.