spousal exemptions to property tax increases upon the death of one partner who is a co-owner of the home;
veterans' discounts on medical care, education, and home loans;
joint filing of tax returns;
joint filing of customs claims when traveling;
wrongful death benefits for a surviving partner and children;
bereavement or sick leave to care for a partner or child;
decision-making power with respect to whether a deceased partner will be cremated or not and where to bury him or her;
crime victims' recovery benefits;
loss of consortium tort benefits;
domestic violence protection orders;
judicial protections and evidentiary immunity;
and more....

Most of these legal and economic benefits cannot be privately arranged or contracted for. For example, absent a legal (or civil) marriage, there is no guaranteed joint responsibility to the partner and to third parties (including children) in such areas as child support, debts to creditors, taxes, etc.