Sheldon Nalos , Nov 16, 2009; 04:57 p.m.
It limits how close the lens can focus. If you plan on shooting exclusively distant subjects (sports), it will help speed up overall operation. You avoid the lost time of having the lens miss focus, then run all the way to close focus, then all the way back out to infinity. If you want the lens to use the full range of focusing (all the way from 1.5M to infinity) then you set the lens to 1.5M. If you want the lens to focus between 3m and infinity, then you set the lens to 3M.
FYI, not sure what Ritz's return policy is or if they'll charge you a restocking fee, but you can get that lens new at B&H for $1220 with free shipping. Ritz's prices have always been terrible (hence their bankruptcy).