We were staying together already before our marriage.... Heh!
I honestly didn't need a piece of paper to tell everyone that I'm sticking it out through hell or high water.
But, I must admit, that piece of paper provides things:
1) security for the woman.
2) legitimacy to the children.
3) social benefits to all of them.
4) acceptance by a judgmental society (in terms of how they view the lady)