normally a mesh node is displayed only with material pins. The Object layer map node offers the possibility to have the mesh node subdivided by objects instead of only materials. In this way we are able to hide a specific part of geometry or assign different render layer numbers to easily work in Render Layer pass mode, or work with material baking, or easily create a custom mask... always without interferring with materials.
It is also possible to display the objects pins without using the Object layer map node, by changing the option in the mesh preferences:
