Frontend designers often extol the benefits of layouts using CSS Grid, but I have to wonder if it's really that important given that people spend the majority of their browsing time on a mobile phone.

While it's now __possible__ to layout complex designs easily using CSS Grid, it's usually not the best user experience on a phone. Why go to the trouble of making a beautiful layout with lots of white space that won't show on mobile without scrolling?

Yes, you can use media queries to give a better UX on mobile. But if the end result is going to be a single-column page that strips out whitespace for the majority of users, why not just use a collapsing columnar layout?