The class-less (or no-class) CSS framework's promise is that as long as you have a valid HTML and structure your elements in a sane way, you will get a decent-looking page by including one stylesheet.
They are useful to put something online quickly. It can also serve as a starting point or as a temporary solution while you finish other aspects of your application or website.
Since they style only the general HTML elements (paragraphs, headings, buttons, etc.), you don't have to worry about adding their unique class names or removing them later.
Some class-less CSS frameworks have a distinct personality, like Bahunya or YoRHa. Others provide minimal styling, focusing on the spacing between elements and making things acceptable. The latter ones are an excellent choice for prototyping applications. The Water.css and awsm.css seem to be good ones.