It helps you identify the best location for your subwoofer. The problem is that might be the side wall, back wall, etc. Many people just can't place the sub anywhere in the room for various reasons.
Basically, you place the subwoofer at the main seating location where you normally sit, either on the chair or move the chair and set in on the floor. Then, while playing a track with good low bass, you crawl around the perimeter of the room, listening for the location where the bass is the best sounding and not booooooomy. From there you just swap locations.
http://www.youtube.com/watch?v=AV3oLLMgS-M
