Not really, fixtures are the difference in position between machine coords zero and the fixture offset of which there are a lot

To simply save DRO position on a shutdown/restart there are persistent settings for DROs on General Config page.
Reason you sometimes see the fixture save option is because you have zeroed (or modified the value) a DRO and so basically have changed the fixture offset. The numbers you see in the DRO will normally be an offset and unless you have called another one then it will be G54. If you press the Machine Coords button you will see different values in the DROs, this is the machine coords position which is set during homing and is the coords that Mach bases everything on.
Hood