D3.js is quite low-level library, it's basically a wrapper around SVG with some sugar like animations and enter/exit/update.

> Is the most apparent benefit less lines of code?

Yes! With "reusable components" you don't need to write (or copypaste) a line chart over and over.

From their demos:

    brushChart
        .width(containerWidth)
        .height(300)
        .onBrush(function(brushExtent) {
        // Do something with the brushExtent
        });

    brushContainer.datum(dataset).call(brushChart);