It defines css classes for most css properties. Their website [1] explains why you might want that pretty well. Regarding file size, the idea is that you use something like PurgeCSS in your build pipeline, which removes all unused css classes. A bit like tree shaking for css.
id imagine that you get a lot of rules when you have a whole bunch of different "margin-top-<x>". the idea is probably to rely on some CSS tree-shaking/"dead CSS elimination" tool to only keep the ones you need