It provides a massive set of CSS classes which allows you to do styling inside HTML instead of manually in CSS. It provides the benefits of consistency and reduces the need to name things which can be a big time sink. It only includes the classes that you use in the final build.
It does have the potential to significantly bloat HTML when many classes are needed for a given element though, and directly opposes the original intent that HTML defines only content while CSS defines styling.
Transmission is awesome because it’s simple. It only does what you need and has the best UI for doing so.