I’ve googled it, searched the forums, and I don’t know where else to find this lol. I know its really random But I’m interested in knowing what the towing power mode switch is for on the b787. I’m sure it’s pretty simple given the name, but thinking more into it, why would you need to supply power during towing? If you’re thinking about the tow part, well the tug doesn’t need power from the aircraft, and if you’re thinking about powering the aircraft during towing, isnt that what the APU is for? Only other thing I can think of is if the aircraft is getting towed from hangars without passengers and such but then again why would it need to be powered during that?

Like I said, super random I know but just want to know. Thanks for any input.  

An aircraft under tow needs to be powered for hydraulics for brake pressure and for the beacon and nav lights and the radios if you're repositioning on a controlled ramp or taxiways. We always started the APU but maybe the 787 has a different feature where you don't have to do that.

As far as I am aware, it only powers the absolute minimum systems necessary for a safe towing operation, in order to save the load on the battery and apu systems. So that would be the nav lights, in order to warn people on the ramp and other vehicles that the aircraft is moving, the intercom, so you can communicate from the flight deck to the ground crew (who are themselves in communication with the airport ground controllers, so the aircraft radios need not be powered up), the brakes, so you can stop the thing and park it, and the lights on the flight deck and in the main aisles, so you can see what you are doing on the flight deck and could also find your way to an exit should an evacuation prove necessary.

As Mike and Alan said, it provides the following:

(BATTERY switch off and no AC power)
• selects main and APU batteries for powering towing operation loads,
including:
• left audio control panel and flight interphone
• flight deck dome lights
• wing and tail position lights
• brakes
• applicable towing battery charge light illuminates, depending on current
battery voltage (HIGH, MEDIUM, LOW)

Note: Radio communication systems are not powered by the towing power
switch.

The 787 has electric brakes so you need to minimize brake usage in towing mode since it can deplete the batteries.

Another factoid is that in this mode the APU battery not the main battery provides the power to the nav lights.