Why don't some airlines assign seats to passengers? Why don't some airlines (for example Southwest in US) assign seats to passengers? How does it help them? Doesn't it delay the boarding (passengers going back and forth inside the plane to grab their favorite seats)? 

Are there any studies to show that this is a better method? Or is there any technical reason why some airlines does this while others do not?
It is mainly used by low-cost single-class cabin airline companies to make the reservation process simpler.