People get married for various reasons. Some people need the commitment, others do it for purely romantic reasons. Others, because there is quite a lot of social pressure from society, parents, friends to have the "big white wedding".
In my particular case, neither my husband or myself were that keen on marriage. As an institution, it is patriarchal, old fashioned, and not particularly fair on women, who have to give up so much of their lives to raise the children and keep the house running. (But that's a different topic, for another thread)
However, once we got our first child, we realised we were doing everything that marriage demands of people, e.g commitment, responsibility towards our house and child, shared finances, with none of the benefits. And when I say benefits, I mean legal benefits. For example, if one of us died, half of the paid portion of our house would have gone to our parents, not our partner. In case of accident, we wouldn't have been the nest of kin or have any rights to make decisions on the other.
We would have had to talk to lawyers and draft wills and various documents to protect ourselves and our child in case of misfortune. It seemed really silly and expensive at the time.
So we got married and it was a lot easier. No big wedding. Just registry office and meal afterwards.
We still have our arguments as before. I still love the same things about my partner as before and the same things infuriate me about him. Not much has changed overall but at least now we have the peace of mind that if something goes wrong, we are protecting each other legally and financially, and that's a huge advantage.