There's a new video in the Gallery page comparing Viewer to ArcGIS Explorer side-by-side, working with the same GDB Geodatabase, the Naperville Gas example that ESRI published.
Explorer is a fine viewer product ESRI had published and developed for several years. It has the great advantage of being able to connect to ESRI GDB geodatabases. Unfortunately, ESRI announced that as of December 2017, Explorer was going into end-of-life and would be replaced by products like ArcGIS Earth.
There's much to like about ArcGIS Earth but, alas, Earth does not connect to GDB geodatabases and thus cannot be used as a viewer for them like Explorer could. This new video shows Viewer compared to Explorer to demonstrate how Viewer connects just fine to GDB geodatabases and, if anything, can work with them noticeably faster than Explorer. ESRI people who want a free way to keep viewing their GDB geodatabases can use Viewer to do that.