Bootstrap is a great base or starting point for web developers with support for cross browsers (Internet Explorer 7 & up, tablets, smartphones), grid based for easier layout, and a responsive design that adapts to the various devices it's on (view the page itself and resize your browser window to get an idea of what that looks like).

It's a great template to use for quick one pager sites that don't require a lot of content to be changed later (i.e. groveyouth.com/ which isn't actually built with Bootstrap, but only because it wasn't out when I was building that landing page) or for a really high traffic site where you don't want a heavy, slow CMS making people wait for it to render out the content.