Here what I understand / know, correct me if I’m wrong.
Vf is the LED forward voltage, this relates to the current going through the LED. Higher current, higher Vf. This relation is non-linear though. It rises fast on the very low currents and then levels out a bit until you get to the really high currents (weird things happen).
A linear regulator “burns off” extra voltage as heat. Thus a fully charged cell sitting at 4.2V driving a low Vf LED at 1A having a Vf of 2.9V (219C approx) will burn off 1.3V at 1A. Thus you would waste 1.3W of power in the driver.
The upside is that the battery voltage can drop to something like 3.2V or so before the linear regulator does not have enough overhead voltage to keep regulation. A higher Vf needs a higher Vbatt to maintain that margin.
And here is where a well designed buck regulator will win, it will not “burn off” the excess power, instead it will use less current from the cell when the voltage is higher than Vf. Of course, some power is lost in the buck converter, but it will be no where near the loss of a linear driver. (If well designed.)