Buttons are sort of a pathological example. Most components in a web app are container-like things, with collections of a couple of buttons and other things that need to have a cohesive look, internal and external positioning, etc. And how often do you end up with one large, crufty CSS class for each of those, or a handful that are closely tied to each other and the markup anyway? Frameworks like this allow you to easily move that gunk from the CSS to the HTML.