It's for the musical benefit of the listening audience. It's been known for a long time that elevating speakers has problems*.
That can be recovered with tone controls and volume knob on the amp or an additional EQ stage, but for maximum clean sound using the least power, the closer to the floor the better...
for the listening audience, not so much for the performer because the instrument's stage tone they hear will sound
thumpy and thud-like, missing a lot of the high end which provides most of the aural intelligibility for identifying and confirming perception of the chord types, melodic pitch discrimination, etc.
Resisting compensating by increasing treble and volume, so forcing yourself to learn to hear your instrument's stage tone without the crispy and clear sound (which takes about a month or two if you have a good ear), the long term ability to actively hear
all music increases in perceptual speed and coherence. This is the fastest and most natural way to really improve your ear if you can make it through the period of struggle (similar to the month or two that our fingers ached when we first began to play).
*Problems, e.g.,,, from the 50s, so read "c" as Hz
Mental check on buying a good guitar
Today, 10:38 AM in Guitar, Amps & Gizmos