Gerrymandering may be used to achieve desired electoral results for a particular party, or may be used to help or hinder a particular demographic, such as a political, ethnic, racial, linguistic, religious, or class group.
Gerrymandering is effective because of the wasted vote effect. By packing opposition voters into districts they will already win (increasing excess votes for winners) and by cracking the remainder among districts where they are moved into the minority (increasing votes for eventual losers), the number of wasted votes among the opposition can be maximized.