Imagine the ability to quickly and easily visualize data coming from any 3rd party systems, WMS layers, ArcGIS Server data, or other datasets, in ArcGIS Online or Portal. Rather than creating a 'join' snapshot of the data, you would have the ability to visualize, query, and view the related data inside of one new 'Feature Join Layer'.
From the My Content, a user could specify that they wanted to create a new 'Feature Join Layer'. They could then point to multiple data sources and define the relationship similar to ArcMap (based on fields or spatial).
The new 'Feature Join Layer' could then be used as any other feature layer, but contain a live 'view' of the data from multiple databases.
This idea has been marked as a duplicate of AGOL relations in feature services.