This has already been briefly mentioned, but ...
ISO 226 equal loudness contours (aka Fletcher-Munson curves).
As the volume gets softer, progressively more bass and treble are required for the tonality to be preserved. Without this type of compensation, music will seem to lose bass and treble, which will rob weight and "sparkle" from music.
Old fashioned preamps had a "Loudness" feature that was rather crude - it was either off or on. Modern playback software with volume control can implement this digitally, so the curve changes continuously as volume is changed. Examples - JRiver, Foobar, Acourate Convolver, Hang Loose Convolver. Notably, Roon does not have this feature.
In the old days, people used to talk about "lazy drivers" which needed some decent current to "wake up", as an explanation for why some speakers did not sound great at low volumes. They typically spoke of larger speakers with massive drivers that "did not like" being played at low volumes. There is apparently some kind of U-shaped curve where drivers perform in a linear fashion. I know nothing about the truth of this, I suppose there is a possibility that a small current can't get all that inertia from a giant cone moving. Maybe someone here can comment.