- The right to make medical decisions on behalf of the other
- The right to visit the other in the hospital
- The right to make funeral arrangements for the other
- The right to survivor's benefits (veteran's benefits, Social Security, private pension, etc.)
- Income tax breaks and credits
- Tax breaks on inheritance and estate taxes
- Tax breaks on money and property transfers between spouses
- Immigration and naturalization rights
- Can't be forced to testify against the other (usually)
- Communications between married partners are privileged from discovery in civil and criminal cases (usually)
- Joint adoption rights
- Bereavement leave
- Joint bankruptcy protection
- Automatic recognition of the relationship by every state, nation, etc.
Etc. There's something like 1,000 rights, privileges, and responsibilities that attach through marriage only.