Apart from helping avoid redundant code, I could not imagine many use-cases for this.

However, more than that, I see it as a possibility for creating CSS that is not dependant on the class-names you put in the markup.

So, instead of using instead of cluttering your markup with classes like

<a class=“btn btn-red price-btn”>Buy Now</a>

and your CSS like

.price-btn {
    font-weight: 700;
    ...
}

You would would be writing

<a class=“price-btn”>Buy Now</a>

and your CSS would look something like this:

.price-btn {
    @apply --btn;
    @apply --btn-red;
    font-weight: 700;
    ...
}

Which, I think is a pretty neat as it separates your markup from additional style-dependent classes.

Could be a nice replacement for cluttering up your HTML with utility classes. https://tabatkins.github.io/specs/css-apply-rule/ This page here has a couple of examples of its use, and thats about what it seems to amount to.

@apply should become the new mixin I reckon, combined with native CSS variables. It will take a lot of time (making specs, getting vendors to implement), but it might remove the use for SASS/LESS in a lot of cases.

Does CSS @apply rule solve any real problems? or are we just making CSS harder and harder with complex/unnecessary features??

If @apply works like a SASS/LESS mixin, it does. CSS will evolve and it makes sense to copy the good parts of SASS/LESS and make that native CSS behaviour :)