Say, I build a declarative tree describing a layout, and I render it. Now I remove one item from the tree. Can the layout engine efficiently remove the item from the view, and... (now it comes) provide an animation for it? As in, the item slowly losing opacity, then the surrounding items moving closer together? (Or a different animation depending on settings).
Of course, the opposite should also be possible (i.e., inserting an item).
Implement this, and we're a step beyond CSS, because so far this has not been possible (elegantly) in CSS. I.e., remove the element from the DOM tree, and it is gone immediately (without the animation). And no amount of CSS can fix this.
We have LayoutAnimation inside of React Native which lets you do that. It's not very well documented and the delete method is not implemented, but the idea is there :)