Why does pulling out main speakers from wall improve sound?

@jumia the floor is only a concern for boundary reinforcement. That can be fixed if needed by equalization (DSP). The front wall reflection comes back towards the listener (not the ceiling). It is critical.  The muddiness is usually a large suckout and some reinforcement around the wavelength = 4x distance to wall. Early reflections is there two but that's imaging not muddy. Muddy could be too much boundary reinforcement emphasizing bass too much.

In my opinion HENRY53 is offering the best and simplest answer and I am hesitant to try to expound.  God gave us TWO ears on the side of our head and a brain that can detect minute differences in the timing of sound.  A voice arrives a millisecond earlier to one ear and our brain can instantly locate the voice in the dark.  Some sounds reflect or bounce of surfaces and others are more likely to get absorbed and diminish. Moving the speakers off the reflective surface of the back wall and side walls allows for better timing of the sound arriving to our ears.  There are a ton of other considerations such as low frequencies attenuating in corners or even canceling out.  That's why i agree when people advise to experiment with location and angles.  

Any sound hits the human ears from reflective surfaces, front or side wall, that is not delayed by 5 millisecond will be perceived as the direct sound. That reflected sound will be convoluted with the direct sound coming out from the speaker and compromises the sound image (and possibly other traits of sound), again, if not delayed by 5 millisecond. The speed of sound is about 343 m/s at nominal temperature of 20°C in dry air. So the sound travel distance in 5 millisecond is 343x0.005/0.3048 = 5.63 ft. Half of this distance, from tweeter to the front/side wall, is 2.8 ft just under 3 ft. So, maintaining a minimum distance of 3 ft is the rationale. I won’t say to give improved sound by pulling your speakers out for that minimum distance, rather to maintain the clean, unpolluted sound. I hope this traditional wisdom from many other experts help answer your question.

Speaker Placement: How far from the wall should I place my speakers? - (gikacoustics.com)

Good video @yyzsantabarbara , it's my post from above but in easier to understand video form. They don't address how boundary reinforcement though and how to deal with it.