Lets imagine situation: you have node event and node article. Event node have no references to article, only article have reference to event. You need to put one event field on each article which have reference to that event. Maybe I am wrong (please correct me if it is not true) but we are not able to do it in views module. There were couple modules which allow backreference, but in fact we don't need to install new module to have this functionality. We can make it in small custom module, lets call it it backreference block. Code example given above.