Don't like un-answered things like this but I cannot find an answer.
One other forum suggest it's good idea for hill climb - incase you fail and have to come back down it backwards in a hurry. Also said they think it makes traction control more sensitive/quicker response.
Hill Descent Control: Uphill
The handbook says on page 115
CLIMBING STEEP SLOPES
ALWAYS follow the fall line of the slope -
travelling diagonally could encourage the
vehicle to slide broadside down the slope.
Select gear selector position ‘1’ and engage hill
descent control (HDC). If the surface is loose or
slippery, use sufficient speed in the highest
practical gear to take advantage of your
vehicle's momentum. However, too high a
speed over a bumpy surface may result in a
wheel lifting, causing the vehicle to lose
traction. In this case try a slower approach.
Traction can also be improved by easing off the
accelerator just before loss of forward motion
Unfortunatly it doesn't say why use HDC