In any real application you will have far fewer semantic class names than combinations of style properties. Working with concepts is vastly easier than trying to remember the specific property differences between concept A and concept B.

Obviously, a real application will have more than one css property. Also, your widgets will share styles, usually in a fairly obvious hierarchical way. And your designers will want them all to be consistent.

In this world, it’s far easier to remember that a widget is “.widget”, and that “.rounded-widget” is for the round version of that”, than it is to remember that the former concept is “.b1p .m5 .ib .xyz .pdq .foo” while the latter is “.b1p .m5 .p2 .br10 .xyz .bar”