A delay stack is used in a PA system for a larger event in a very long room. In a long room the quality of the sound deteriates as you further down the room as sound is absorbed by people, carpet and furniture. Of course you can add speaker halfway down the room, however sound travels at approx 340 m/s or approx and the sound will be out of time. The sound from your front speakers will have an echo compared to your mid speakers and your event will sound like its a train stati