Focus adds nuance of being able to still be connected to work with certain apps and to be in reach of certain people, rather than a binary ON/OFF approach that DND and AirPlane mode provide.This all seems like a lot of effort in order to do what Do Not Disturb and Aeroplane Modes are supposed to do.
Maybe that isn't useful for you, but that doesn't make it a poor feature. Based on early indications, many will get a lot of value from it.