No doubt this is to get a selection map to make it faster to determine what object was clicked on in the viewport. See the custom rotation plug-in @kitjmv has been working on. It would eliminate the need to use GetRenderMeshes and do expensive ray hitting he has been asking a lot about recently.
And some of the related posts RhinoObject.GetRenderMeshes, bug? , Render mesh topology , How do I always switch to wireframe when moving the view? . I thought there were more, but can’t find them now (at least a post with a huge amount of objects in and trying to get render meshes and do fast ray intersection and determining overlapping objects).