It's for the legal benefits, yes, but it's also a title that gets more respect in our community. 'Husband' means more than 'boyfriend' when used to introduce someone.
To me it's also a religious reason, and it's a promise made to both your partner, your government, and God. To me it seems like something that will help to hold you together when the going gets tough, just like an extra little hand hold.
That's just me, though.