What would the reason be for this helicopter maneuver?
Ex-Royal Air Force helicopter force here, spent a lot of time around Chinooks.

I think the main answer is "because I can", but I can think of a legitimate combat use.

You are heavy, and forced to land downwind into a narrow strip because there is a hazard on the upwind side of the landing zone.  There is not enough room to turn around.  You could hover taxi backwards like this to pick up some inflow into the rotors from the relative airflow (and therefore increased lift), then climb away backwards until you do have enough room to transition into normal forward flight, more or less as shown in the video.

This is where team work kicks in - you've really got to trust your load master who is telling you exactly where you are.

Never seen it done though.