Void/Illegal Marriages
Void or illegal marriages are marriages that do not exist in the eyes of the law. Spouses in a void marriage do not have any spousal rights. For example, the wife of a man already married cannot claim her husband's estate when he dies. Since he was married at the time of their marriage, the marriage is void.
Theoretically, these "non-existent" marriages would not require a judicial decree dissolving the marriage, but obtaining an annulment resolves any legal issues that might arise regarding whether or not the parties are single and free to marry again.
SIDEBAR: Same sex marriages are void in some states. For instance, the California Supreme Court has declared marriages that occurred because a license was issued in San Francisco invalid.