I don't quite get what the big deal is with @scope:

What's the benefit to:

    @scope(.card) { 
        header {
        }
    }

over

    .card header {
    } 

(or ".card > header", the article isn't quite clear which it is). Is it just the sass-style nesting? How does it differ from @nest then?

On the surface, it's very similar to the snippet you pasted and nesting but with a few exceptions. I asked the same and this breaks it down well: https://css-tricks.com/early-days-for-css-scoping/