In practice this means much less texture re-creations and much less redrawing.
The main reason to do this was to unlock using offscreens for more persistent visuals. E.g. it's fine to do some extra rendering for a 150 ms animation, but it's not fine to do it for something that can last seconds or minutes.
In particular, I can finally make windows semitransparent while dragging them in the tiling layout! Which needs an offscreen since it's a stack of border + window + subsurfaces.






