To put your players in a better position at the snap of the ball. If you called a cover 3 and wanted your short-zone safety closer to where he's going to cover, you could roll the coverage and have him down. This can put your players in a better position to make a better break on the ball when it is snapped and put another man in the box. It also takes the CB's far off the line which can guard against fades. Downside: giving away coverage, taking men OUT of position if you are disguising something else.