Even if you can't pass an argument, this is a big step forward. For all the CSS frameworks, where you'd do something like <p class="grid-1 col-m bg-2 wd3 pd-m fs-1"> and the semantics go out of the window - with @apply I can use semantic class names such as <p class="important"> and then map .important to all the CSS framework visual classes in the stylesheet.
Presumably I could do that without needing a pre-processor, keeping a lean workflow and preserving ability to modify the stylesheet directly from Chrome Devtools.