After installing ArcGIS Enterprise (aka ArcGIS for Server 10.5) an Azure LoadBalancer instance is created, in addition to a configured ESRI webadaptor being installed. It's unclear (not documented) what role the LoadBalancer plays and how it works with the ESRI web adaptor. It seems that they potentially do the same thing which is load balancing. Below is a screen shot of a single server install. In a multi-site install the contents of the resource group are the same with the exception that there are more hosts. The creation of the LoadBalancer is new to 10.5, what is it doing?
This question is covered for AWS and can be found on an ESRI FAQ titled Is ArcGIS Web Adaptor installed when I create my site with Cloud Builder? but a similar answer for Azure does not exist.