Shai
Nothing. It's a waste of time imo. If you are bored in game then do the quest. I didn't do the quest because I think it is the same as 2 years ago. I did the quest then. I got the couple ring where you can 'engrave' the name of your couple in that ring. Which basically just adding your couple's name on the item description. Also, it allows you to teleport your couple all around Madrigal just for a single use (if I remembered correctly). For example, couple A is at Flaris, by double clicking the couple ring couple B will be teleported beside couple A wherever couple B might be. As far as the stats/effects of it, there is absolutely none. Do it and try it. I don't think the quest is that long and tiresome either.