Allowing access to geometry and tessellation shaders would allow for modders to create highly detailed larger terrains in a manner similar to Outerra. This would be useful for modding in general, and would not just be limited to terrain.
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That's not the point of this, and that could be accomplished using pixel and vertex shaders/the supershader. The point of this is to allow for modders to create custom materials that use the geometry and tessellation shaders to do things such as create procedural geometry on the GPU, similar to a normal map.