I'm guessing it's done on approach so that the autopilot won't keep the plane from landing -- which would beg the question of why the autopilot would even be on during takeoff or landing.
As one would guess, it shouldn't have been on. Insight comes from a comment in TFA:
"I’m a captain at a U.S. major airline with a lot of time in 777s. Something this doesn’t mention is the that even if ground level was set into the mode control panel, the aircraft wouldn’t have descended if it was being hand flown. That means that the flying pilot here selected the autopilot on immediately after takeoff. This is unusual at Western carriers as we like to hand fly. To me this indicates a lack of experience or confidence or both."