People always say tax benefits, but have you found that to be true?

AFAICT, the purpose of marriage is to bond two people so that it's harder/less likely for either of them to separate (at least in the short run), thereby giving each person the comfort of security.

Social habit? Everyone was doing it…

A legally binding declaration of love that puts you into a life long contract with one another. I guess it helps for tax purposes and it simplifies the surname of any children.

It was historically a social construct to keep family together in order to legitimise the ownership of the family wealth and powers. Also it secured more or less the survival of girls and women in their new families. But there are and were other social constructs that did the same purpose. Like e.g. family tribes, polygamy. Also some people had the habit of sending their children away to more or less loosely connected relatives or friends who formally adopted these children as part of their family. It was a method to socially bond beyond biological family ties and an effort to increase survivability of family members.

No purpose.

Trump getting richer.

well it's got the social and cultural purpose to 'legitimize' an intimate relationship, it's also a contract with a government to recognize a non-familial relationship with regards to things like benefits and inheritance and such... it was originally a mechanism for families to merge their money and power and increase their influence and whatnot, I suppose love is in there somewhere, too

Business.