> What good use cases would you see for grid lanes today?

Fully responsive layouts, where sidebar content is interleaved with page content on small screens, but in a sidebar on larger screens.

Demo: https://codepen.io/pbowyer/pen/raLBVaV

Reordering the content on larger screens would be the icing on the cake but for now I'll take just doing it.

CSS Grid didn't solve this, as it added gaps: https://codepen.io/pbowyer/pen/azNarbZ

And using named grid-template-areas stacks the items you move to the sidebar on top of each other, so you only see one of them at a time. Eventualy I hope that https://github.com/w3c/csswg-drafts/issues/9098 will land and we'll be able to use this saner way to do it.