Making nice looking collapsing columnar layouts gets easier with Grid as well (I've played a bit with it, and a few things that required horrible hacks before now are really easy). You also get a lot more flexibility in how you order things, which is nice both for responsive layout and things like screen readers.

And not everything has a so vast majority of mobile users that optimizing the experience for desktop/tablet users isn't worth it - especially for sites/SPAs with many elements and very complex layouts.