A standard feature of NSToolBar (the same interface you find in Finder, Xcode, Mail, etc.) is the ability to customize the menu. What may not be so obvious is that Quartz Composer uses NSToolBar for the Editor as well as the Viewer, as this is undocumented functionality.
By Command+Clicking anywhere on the QC Editor toolbar, or the Viewer, one sees a menu with the option to "Customize Toolbar...". Choosing it will open up the interfaces pictured above.
No "superpowers" are gained; all functions that are gained by both methods are available through the QC menu or hotkeys. However, there is some convenience and speed or workflow to be had.
Most Editor addition possibilities are useful, save for perhaps Customize, which simply invokes the Toolbar customization menu the same way that Option+Clicking does. The only utilitarian addition to the Viewer is the ability to take Snapshot directly from the Viewer, instead of from the menu or hotkeys.