G92 is used to set position after manually moving the machine. A number of people with no endstops use it for homing (adjust position by hand or with G1 commands and then issue G92). It's used to restart a partially failed print by setting the last known printer position. It's also used (on E) to set filament position to a known value after changing filament mid-print. Other than the restart function all of these are possible with x=y=0. That limitation on delta would be better than making it a no-op.