hello,
i am not a 777 rated pilot. just flying the baby boeings (737s) but i can think of a reason for that switch. i may be absolutely wrong about it though.
recently our recurrent training has an exercise of landing gear will not retract after takeoff. possibly due the landing gear lever/handle selenoid (i guess 777 may not have one) or the air ground sensor still stuck in ground. thats a pretty tricky one as loads of things wont happen even though aircraft is airborne. LNAV/VNAV wont kick in, there wont be automatic thrust reduction, aircraft wont pressurize etc etc, as the aircraft still thinks its on the ground.
maybe in 777s in that situation a single button press can bring the A/T in clb or con mode. who knows maybe it will even let VNAV to kick in also. as in 737s in that situation we are not LNAV/VNAV capable anymore.