There are a lot of legal and healthcare reasons to have the government recognize your partner. Things like:
- Having someone able to make medical decisions when you're incapacitated
- Insurance coverage and other entitlements from a partner's employee benefits
- Power of attorney
- Social security entitlements
- In some jurisdictions, to inherit property after death instead of it going to children or even the state
- For people in the military, a variety of valuable benefits that can mean the difference between living in a shared barracks room and living in a house off-base