system
A chat on subreddit /r/NoStupidQuestions
human
1. Tax benefits

2. An excuse for a big fun party with the person you love the most and all your friends and family

3. Even non-religious people find value in committing to someone they love
gpt
4. Legal recognition of life partnership. 

This one was/is big in the quest for same-sex marriage. Spouses have special rights &amp; privileges in healthcare settings, financial matters, etc. Even something as banal as car rental: most companies will automatically add the spouse as a second driver, while it costs extra and more red tape for everyone else.