In the USA there are tax benefits for having a contract of marriage with some one. There are also legal benefits. For example, under
fifth amendment rights one is not obligated to make incriminating testimony against one’s self if charged with something in court. This benefit also extends to one’s spouse (so a husband cannot be forced to testify against his wife). But if two people are not married (or they are married, but end up in court in a state that that does not recognize that marriage) and one refuses to give testimony in court that may be incriminating to ones spouse penalties for obstructing justice could be applied.
Medical benefits through an employer can be used to cover medical expenses for off both offspring (or adopted children) and one’s spouse. When the marriage is not recognize the medical benefits can’t be shared nor can joint adoption occur. For an adopted child to be covered by the health benefits the parent with the health insurance would need to be the one adopting. Understanding the implications of those might require understanding how the evolving American health care system works and fails.
There are some other matters in the real of legal rights and benefits that are only accessible through marriage in the USA. Can’t go into all of them here