Marriage comes with various legal/financial benefits.
So plenty of reasons, depending on circumstances. Seems a bit naive to even pose the question tbh...
Also marriage predates the Middle Ages btw... it certainly isn’t a “medieval concept”.
I’d agree with that, although with my first marriage, in 1961, I can’t remember how it went, one minute I was a regular 20 y.o. guy, raining kisses on my inamorata’s lips, and walking home, still with an erection that took a couple of miles to subside, even in pouring rain or ice and snow, next minute I was a 21 y.o. husband, Christ knows who organised the cars, flowers, hall for the reception, and everything, although I vaguely recall buying her wedding ring at a jeweller in Deptford.
My next long term involvement, after having quite rightly been divorced nine years later, lasted eight years, with her occasionally hinting that her Catholic parents would like us to get married, but with me trotting out the old saw about, “who needs a piece of paper? We love one another, don’t we?” Then one day I came home to a house full of furniture, but no common-law wife.
Plunging in for definitely the last time, in my mid fifties, I realised what a dog’s breakfast it would be for her if I cashed in my chips without marrying, sorting out my divorced parents affairs after they died alerted me to that, and at nineteen years my wife’s senior, it was a million that I’d go first.
So once and for all, I’m a happily married man, again..........