If we speak about classical family – most important difference is a guarantee of  security and full fledged life to woman (and children). Socium is a big mechanism and sometimes one person is to small in it, so legalisation of marriage is a means to secure weaker side of a couple – a woman (and children).

Other reason of all this is that men are unpredictable beings and, they die faster (earlier) and sometimes they just disappear because of many possible reasons.

So if woman side is not secured – she can’t life full fledgedly and bear children.