I had a case recently, where I needed to add custom data to the node display and wanted this data to behave like a field, however the data itself didn't belong to a field. By "behaving like a field" I mean you can that field at node display settings and able to control it's visibility, label and weight by dragging and dropping that field.
So, as you may have undestood, hook_preprocess_node / node_view_alter approach alone wasn't enough.