A marriage license indicates that your marriage is legal. Implementing this system of licenses help ensure that the marriage is mutually decided between the couple and/or their families. This helps to discredit illegal marriages, also it does little prevent them at all. This license is important in some countries as married couples can sometimes get more benefits than other citizens.
As CUEVAS and BYRD54 have stated, a marriage license is legal proof of your marriage's existence and is a document that might be required for any governmental processes that might require it. For example, in Singapore, potential buyers of government built houses must be married and produce a marriage certificate before they are allowed to purchase a house.
Marriage licenses are a tool by which the issuing judicatory can create and maintain ‘vital statistics” filings for a county, state, and federal government. Marriage licensure establishes the “legality” of marriage, and are used to insure proper assessment of real propriety, income, and the like for appropriate taxation. Marriage licenses are also referenced after the death of a partner, as they can help probate courts, insurance providers, and the like to determine where to vest survivor benefits and real property.