European carrier Finnair is now weighing passengers at the gate before they board the plane as a way to estimate how much the jet will weigh at takeoff. But it's not mandatory.
Airplane fuel efficiency is impacted by how much weight is onboard, and that includes fuel weight.
The more they know about the actual weight of each trip, the more fuel efficient they can make each trip.
HOWEVER, even though this is well intentioned, it's not hard to see American carriers would love this be standardized. They would definitely leverage it to further increase their revenues. Which they will then reinvest in...checks notes...stock buybacks.