It doesn't have to have any point for any individual. I think it binds people up into nice social units for societal control. By societal convention, you have t

It doesn't have to have any point for any individual. I think it binds people up into nice social units for societal control. By societal convention, you have to marry, just not by law (probably because you or your family historically chose a partner, and it would look quite obviously compulsory if the government mandated a date and a partner.) The government only nudges you that way with things like joint tax filing options. The laws have gotten less strict, and the social stigma lessened, but the state tries to keep it together as well. You mentioned that a piece of paper won't keep them together, but in most US states, "fault" for a marriage breaking up still comes into play in things like deciding how a court will divide assets, etc., and historically, sometimes you couldn't even get a divorce without someone getting assigned fault.