Useful in airliner-derivatives with split-axis autopilots during military AAR operations as it allows full manual control of the angle of bank and roll rate during turns on the AARA.
But more modern aircraft have smoother AFS response in HDG mode, so that is used by some operators - NAV mode giving little warning of the onset of a turn.
The A330MRTT may have a modified HDG mode to reduce rate of change of bank angle when entering the turn, approaching the desired bank angle, rolling out of the turn and approaching wings level again. This is intended to make the task of receiver pilots easier when remaining in contact on wing hoses in the turn.
In the VC10 tanker, we had the 'MAN' mode for the autopilot turn rate, this ensured that the pilot had direct control of the bank angle. But this feature wasn't fitted to later generation airliners - it being assumed that HDG or NAV would be sufficient.