Marriage has benefits at various levels:
Advantages: Companionship, support.
Can couples live without getting married? Sure, but then there is no guarantee that one of the partner's leaves or has multiple relations. This causes insecurities and lesser emotional commitment. Emotional fulfillment is primary for happiness and marriage is a pretty good practice for lasting happiness. Marriage is nothing but emotional commitment re-enforced by societal customs and legal provisions.
Of course, unless marriage is approached with the right attitude by both partners, marriage by itself alone cannot assure happiness. Marriage can be drudgery without emotional content.
Stability, reduced competition among males and females and hence more time devoted to productive/recreational/creative activities. Else every male will be fighting each other for females and productivity will be low - science, tech, medicine etc.
Even females will be competing with each other for the dominant male.
This borderline anarchy is not conducive for child rearing or for other pursuits of life.
Marriage enforces certain discipline at personal as well as societal level. Of course, there are disadvantages of marriage too - especially of incompatibility, abuse, coercion etc but in my opinion, even today, marriage is on the whole more beneficial.
It is possible to have close relationships without marrying too but unless all partners are mature, somebody or the other will be hurt or loose out.