Why does pulling out main speakers from wall improve sound?

The qualitied among us can give the technical/physics lesson.

It just does, so DO IT! Most speakers do best when given the breathing room.

There are more minute sounds in music (like reverb trails) that need some space to develop and if you’ve got the speakers too close to the wall they get squashed and you’ll lose that sense of space/depth that those signals help create unless your speakers are specifically designed to be near the wall.  Pulling speakers out can also greatly help with achieving optimal frequency balance.  My speakers are about 6’ out from the wall that for me results in optimal frequency balance along with 3D imaging/soundstage.  The good news about experimenting with speaker placement, unlike most things in audio, is that it’s free!

Almost all speakers emit sound in 360 degrees though the exact ratio of the signal behind the speaker to in front of the speaker varies a great deal from speaker to speaker.

When you move a speaker closer or further away from a boundary (i.e. wall) these things change:

• Ratio of bass to mid and treble
• Time from first arrival signals to reflections from back wall
• Ratio of output of first arrival to reflected signals

So, that’s why. :)

The brain can judge distance from sound specifically its reflections, speakers close to a wall create reflections very close in the time domain to the original sound this means your brain 'knows' this is a small space. Speakers further in the room have a longer delay thus the brain hears a bigger space.

Nearfield Listening. Mine are 9 ft off the back wall. For those who have never played with it, I highly urge you to step outside your box and give it a try.