The SINGLE most common use for such grounding bushings is at the terminus of an EMT, GRC, IMC, Flex shielded run on a GEC -- wherever its headed.
Sometimes I've installed them as a grounding lug even with a naked bare GEC system bonding conductor.
Other than that, they've got little purpose.
They are never used to replace lazy-lugs/ lay-in lugs.
And they are typically UL listed for conductor size -- maxxed at 3/0 -- the largest GEC from the table.
That's one hint from the manufacturer/ UL as to their purpose -- they're choke eliminators -- and the inspector will be looking for them on all GEC bonding runs that are wrapped in steel pipe/ steel flex.
They are NOT required if the same run is wrapped in PVC.
As for GEC Systems, the NEC Handbook has sweet illustrations. Exhibit 250.32 [ BTW, the bushing is normally attached physically to the raceway -- but it doesn't absolutely have to -- see the diagram.
Happy to post the citations for the forum...