Well, think of throw as the reach of the orbital polisher. The larger the reach, the more surface area it corrects in one swift motion. This not only reduces your polishing time significantly but it also enables your product to be spread across a larger area resulting in less caking of your product on the pad.
Anyone who has done detailing understands how annoying slinging is (slinging is when product gets spun of the surface of the pad onto the car/environment as a result of the over-saturation of the pad). As a result of this larger surface area, we are able to reduce the amount of slinging that occurs.
The illustration below shows the total magnitude of the throw calculated off the center of the polisher backing plate by adding the displacement from both opposite sides of the pivot point to give you the total throw (ie: 8mm; 15mm ; 21mm).