The Haas or proximity effect is a method of achieving an apparent localization of sound using delay.It is a very useful tool if you are trying to increase the level of sound a distance from the sound source but still want to maintain the illusion that the sound is coming from the source.

Essentially, if the brain receives two identical sounds coming from two separated sources, it will localize the sound to the source that reaches it first, even if the sound it hears second is several dB louder. This effect works best with delay times below 35mS. Above this, the sound becomes more muddied.

Consider the situation where you have to put speakers underneath a balcony in an auditorium so that the people sitting at the rear of the stalls could hear (see the musical theatre system). These under balcony speakers would need delaying anyway, as the sound from them will arrive before the sound from the stage and the stage speaker system.
However, if the sound to these speakers is delayed beyond the point where it arrives at the same time as the stage sound, i.e. it arrives after it by less than 35mS, then the rear stalls audience will still feel that the sound is coming from on-stage.