Sometimes the geometry nodes modifier does support mapping and sometimes it does not. We have no infrastruture to determine this ahead of time currently. In order to support common use cases, it makes sense to add this flag to the modifier. One such common use case is to use the mesh as surface that other things are distributed on. Often, the distribution is controlled by vertex groups. Therefore, it would be helpful if the modifier is evaluated when the object is in vertex paint mode. This allows the user to see the distributed objects while painting. If the nodes modifier transforms the mesh in any way, vertex painting might not work as expected anymore, because the `deformMatrices` callback is not implemented. I'm not sure how this can be solved nicely, yet.