I wonder why the Renpy devs added that feature? Like you said everything is pre-rendered in Renpy games.
Because of its
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. While the name is misleading (it's not rendering), it's still 3D manipulation.
Behind the use of Live2D and the shadering part, it also permit to transform an image into a 3D plan that will have a zero thickness and can be manipulated and transformed on the 3 axis. To this have to be added the "camera" feature (don't remember where to find it in the doc) that permit to change the point of view, here too on the 3 axis. The use is still limited, but it open the door to some possible future evolution that would rely more on that 3D part.
It will never be meshes based 3D, but there's a lot that can be done with plans placed in a 3D environment.
By example, a game like
Sakura Dungeon have an old school 3D dungeon, with just plan sized and placed to represent the walls. This would be easier to do now.
At that time, the game had to compute the deformation all by itself. With the current Model Based approach, it would just need to compute the position relatively to the camera (the player), Ren'Py would then take charge of the deformation. And who know, perhaps that in the future even the computation of the position could be managed by Ren'Py.