Why does pulling out main speakers from wall improve sound?

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.

interesting question. because, with very few exceptions, speakers are designed to be listened to a good ways (1/4-ish a ways) into the room.

very few speakers are meant to be against a wall. boston acoustics floorstanding speakers, like A150, A200, A400, etc are among those designed to be against a wall.

here are some basic rules (there are exceptions) for for speaker placement:

1. your speakers are made to be placed away from the wall and well into the room, about 1/4 into the room away frmo the wall. yep. youre welcome to move em a good distance away from the wall and move em back, u know. its ok. lots of people do this,
2. theyre made to facing the listener
3. with nothing between em. you know all those pics of our gear on furniture sandwiched between two speakers? thats wrong, flat wrong. its as incorrect as pointing the speakers away from the listener is flat wrong or pointing em towards a wall or down at the floor. it makes a great display, but its wrong.
4. some speakers sound and image best with proper toe-in.

because your speakers are designed to be away from the wall and well into the room, pointed towards you with nothing between em or immediately at their sides, they wont function properly if theyre improperly positioned.

here are the ways they wont function properly if improperly positioned:

1. imaging
2. sound stage
3. frequency response

4. dynamics

and plenty of other ways i imagine

Depends on the speaker design. My Apogee full range ribbon speakers are dipole. So sound comes out of the front and back equally. For this reason, they should be between 3-6 feet off the back wall. The entire range of sound sounds better, more air between speakers. Improves in every way including imaging. Bass response also improves.

the other thing you might want to try is angling the speakers at 45 deg instead of having them straight out from the rear wall. I have done that to reduce room reflections and standing waves. Works well in my small room. Moving them out from the rear wall, as others have stated, does improve imaging for sure, and rear wall reflections.