Double hung windows have a sash stop to keep then from opening all the way by a few inches. The only reason I can figure they are used is to make the window operation foolproof. Without it, when Clement Clarke Moore throws open the sash on Christmas Eve, the window could get stuck open. Is there some other practical purpose? Google isn't helpful at all.
Generally, double-hung window sashes are removed by opening them to the limit of their travel, then pulling the sash away from the frame (after removing sash stop and molding). This movement to the limit also usually reveals parts of whatever balance and attachment mechanism(s) the window is equipped with. "They" don't want the end user to extend the window that far; except deliberately, to effect repairs.