They are required for ungrounded systems. A 240 volt delta secondary for example.
A 600 volt from a wye connected 4 wire secondary requires you bring the grounded conductor to the service box and bond it and ground it. Thereafter you don't need to carry the neutral unless 347 volts is needed. The ground detector lights are not required in the second case as any phase conductor contacting a grounded enclosure will pop a fuse or breaker.
