Living Without Marriage
As Irfan sahab has already explained, the contract of nikah provides the protection for both husband and wife as well as future children. Societal norms have no validity in matters that have been clearly expounded by God.
The wisdom is to make sure that a man and a woman are in a formal bond of marriage, not just for fun. If such uncommitted relationship is allowed, it will ruin the institution of the family. The loyalty that the relationship between a husband and a wife demands requires a formal agreement. Signing a legal contract is another way to formalize and solemnize a marriage.