I think the definition you cite is a pretty darn good one. I would add "emotional support" to "emotional intimacy." I know ratbastid is the person I feel most comfortable with, and I am most able to be myself around him. And with him, and with D (although we're not married) I have certainly uncovered (and "uncovered!"
) a lot of my true self. Mostly they push and challenge me to be my best self, and support me in finding my happiness.
There are all those other, less philosophically noble reasons, too - social acceptance, legal privileges, sex, financial support. etc. The best reason I've seen for actually making it legal/public and not just an agreement between you and your SO is the declaration of your relationship to society - somehow it defines your relationship as something permanent and declares your intention to stay with each other to people who support you in that.