Why does pulling out main speakers from wall improve sound?

There is no Wrong....you place them where they sound best to you. Watch Michael Fremers first video on you tube. He was asked why his speakers are so close to rear wall....he says because pulling them out into the room is bunk. He claims his mega million speakers sound just fine as is. My speakers front baffles are maybe two feet from the rear wall if you got a tape measure....they sound great. They are also positioned with lots of toe-in, as recommended by Tannoy.

I think to many people get hung up on what they see and do not listen.

My Sonner Legato Unum stand mounts are 3.5 feet off the back wall. And my subs are not stuffed in a corner they are 6 inches behind my speakers. Sound is great.

There is not right or wrong it is what sounds good to YOU!

Suggest you’re dealer purchase Paul McGowan of PS Audio, The Audiophile’s Guide, The Loudspeaker. After he reads it then he should let you borrow. At least he was honest with you I guess. If he’s not up for it then you might want to order for yourself. 

I would think that there are probably some speakers that are more forgiving of placement than other speakers. . . .

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.

This. It’s all acoustics - treble/staging is very dependent on how pure the initial sound wave is when it hits your ears. Further from the wall, the back reflection is sufficiently delayed that your ears/brain can distinguish. Too close together (too close to wall), you can think it’s just "muddying" the wave.

I believe most know about the lower frequencies being less directional and therefore less depending on room placement (think sub anywhere in the room). For these less directional bass frequencies, room modes are more the driving factor (frequency response, or "boominess" vs "suck outs").

...and maybe look for another dealer (or at least confront the rep [who hopefully isn't the owner])... :-)