One of the very common operations programmers do in ArcObjects is adding a layer to the Map. To do this there are two ways, one using IMap.AddLayer or IMapLayers.AddLayer.
IMap.AddLayer adds the new layer on the top of all the layers in the map. This is not very useful as we normally want the point layers at the top followed by polyline followed by polygon layers. Adding polygon layers at the top would mask all other layers below.
IMapLayers.AddLayer is preferable because it has an option to autoArrange the newly added layer in the table of contents. If autoArrange is set to TRUE, the layer is added in the proper position as by its weight specified via ILayerPosition::LayerWeight. By default, this means that the layers are sorted by layer type – Annotation layers on top, followed by Point geometry layers, Polyline geometry layers, and at the bottom Polygon geometry layers.