Depending on your rig there may be several reasons to have AC-Power (not Air Conditioning) on while driving
For example... In my coach there is a Dish-Pro Dual DVR and a Motosat T2-Automotion antenna (on roof) so that television shows can record while I'm moving from place to place (can't watch and drive at the same time, though I've seen people who tried...... And I've seen their cars... I worked 25 years as a police dispatcher and 3 more as a tow truck dispatcher.... Get the picture)
These devices only have 120 volt cords, no 12 volt
I can, if someone else is driving, retire to the back room and watch in motion tv
I have other 120 volt hardware in the rig as well
All this stuff takes 120 volts, and I prefer not to shut it down when driving.
I also have an Xantrex Prosine 2.0 inverter/charger, this takes the 12volts the vehicle engine's alternator puts out, and converts it to 120 volts so I don't need to run the generator 24x7 if parked where there is no ac, or if going down the road.
There are other reasons to use the generator as well, but keeping 120 volt stuff powered is #1
And my dash air is out of service so I need those rooftop units