compensated tail piece?

I just noticed an ad to the left that showed a "adjustable compensated tail piece" (http://www.mikepecanicmusic.com/catalog_adjtails.html)... i'm rather confused as to the point of this. what's the point of adjusting the string length below the bridge? and if there is a significant adjustment to be made, should i consider adjusting my tail piece up or down to get an "optimal" distance from the bridge?