Type safety. The render object tree can be more type safe since it
can guarantee at runtime that children will be of the appropriate type
(each coordinate system, e.g. has its own type of render object).
Composition widgets can be agnostic about the coordinate system used
during layout (for example, the same widget exposing a part of the app
model could be used in both a box layout and a sliver layout), and thus
in the element tree, verifying the type of render objects would require
a tree walk.

Infinite scrolling

Infinite scrolling lists are notoriously difficult for toolkits.
Flutter supports infinite scrolling lists with a simple interface
based on the builder pattern, in which a ListView uses a callback
to build widgets on demand as they become visible to the user during
scrolling. Supporting this feature requires viewport-aware layout
and building widgets on demand.

Viewport-aware layout