It's the reduction in mechanical stuff (weight, complexity, failure mode analysis/risks etc) that come from not having to create a back-driving system for a system (FADEC) that doesn't otherwise actually need it.
Not sure what you mean by backdriving. On a Boeing, the A/T computer commands thrust lever movement and EEC/engines respond to that movement.
The pilot can intervene with or without A/T disconnection. i.e. a simple chain of command with the pilot always in the chain.
I'm sure if Boeing put their minds to it, the thrust levers/clutchpack could be made as small as Airbus thrust levers (so no weight disadvantage). The servomotor is not what you would call heavy.
Rgds.
NSEU
P.S. Sorry for the thread drift.