Why are bass drivers elevated in floor standers?

Seems like nearly all floor standing speakers made in the last several years elevate the bass or mid/bass driver to about knee height leaving a large empty space below. Does this make a significant acoustic improvement?
I've done this for years,even before ARC.To much air may vibrate the tubes.I also mount my muffin fans so they don't add vibations to the amp.My tubes last long,and I
even use tube amps for tv audio a lot of the time too.
There is also directional,and audible information coming though a lot of woofers in a speaker system. Some that have sub-woofers in them are mounted on the side of the cabinet,most likely because they have lower crossover frequencies that you may not hear if they were directed at you.My guess is,there is audible music coming from these woofers.That would be another reason for this elevated mounting.
The midrange driver is placed very close to the tweeter to prevent uneven frequency response with peaks and dips due to phase problems close the crossover frequency, where the frequencies reproduced by the drivers have some overlap.

Tweeter at ear height, mid right next to it. If the crossover frequency is low enough, then the bass driver can be placed farther away, such as on 3-way systems with the woofer on the side, back, or on the bottom.