Re NOK. It matters in regards to the Mental Health Act.
Being married allowed me to 'pull rank' when my DH's cancer had spread to his brain (in layman's terms), against his anti-vaxing/big pharma/non believing in MH care Family.
As someone's Adult Child, you are the one who can start proceedings for medical negligence etc, unless there is a Spouse.
If they get to the Hospital first they can claim the body and have them buried.
I know a cohabitatee who didn't get a share of an industrial accident payout, because the divorce hadn't gone through.
Marriage is a massive contract, in whatever form it takes.
If your wills are water tight, it solves some of the issues of inheritancy, but a will can be changed or not made at all.
Nowadays the likelyhood of severe disability or death is via a road accident, but it could also be terrorism etc. If you are both injured together, he dies, you don't get to claim his compensation. You may not be able to work again.
I've seen many scenarios over my lifetime to see how marriage benefits both parties, even if they started out as not being the one with money.