The AddLayer Widget for ArcGIS Web AppBuilder allows users to add pre-defined map layers to the current map without having them to include them in the current web map. This provides an easy way to add ad-hoc layers to your web app.
Unlike the AddService Widget which requires the user the enter the url of the service, the AddLayer widget only allows users to display additional map layers that have been pre-defined by the site administrator.
Using the Widget
The AddLayer Widget is an in-panel widget that will be displayed in the toolbar of your WebApp Builder application. Click the icon display the list of available layers. Click each layer you want to add to your map. These layers will be displayed on the map and may be toggled off or on using the standard LayerList widget. You can unclick the layers you do not want to show any more to remove them from the map.
Each entry is the list is pre-defined when the configuring the widget.