What exactly is the purpose of marriage anyways?
Tack-on to 'compete' with Fable and BioWare is my best guess.
In-game, it can provide you with a place (or another one) to live. If you sleep with your companion nearby, you get Lover's Comfort, a 15% skills gain for 8 hours buff, a ring with a 10% less mana cost to Restoration buff, homecooked meals, and if they move into your place, they'll 'open' a shop- you can purchase from them (from all the equipment you gave them before T_T), and 100 gold profit every day from said shop.
My married companion is glitched into perma-block mode, so YMMV on how useful it is.