Pretty much everything the first responder said, right down to the reasons in the past, with the added bit that when the whole thing first started up, it was as much of a way for the male part of the partnership to claim obvious ownership of the female, since women were still considered property at that point.

Currently though, at least where I'm from, it's both an expression of love and intent to stay with the person for the entirety of both of your lives, as well as a legal contract to ensure that if one of you dies, the other person can still retain all rights to any property and money left behind by the deceased.

Lot of people don't bother with it, though, even though there's still a large social push to make it the 'normal' status for co-habitating adults of opposite gender. There are other legal ways to make your significant other the recipient of your things if you die before them. And a lot of people dissolve the partnership after time, due to differences that they either can't or won't work through. That, too, though is a development that's pretty recent; used to be really frowned on if you left your spouse.