Why do I marry?
1) It is more pleasant to call yourself a wife, and his - a husband, than a cohabitant / cohabitant.
2) to sign a single paper certificate of marriage is easier than
a) with each born child to travel to the ZAGS must be two, pay a state duty for establishing paternity, etc.
b) with each major purchase to rewrite a will. (All people are mortal and suddenly, and then to share jointly earned property with his relatives - well his naphig)
3) do not have to long and boring to explain to all the thirsty why we are not married and present more papers in all instances
In general, when you feel OBA that you want to live together all your life (well or most), the stamp only works to ease the bureaucracy, and if before marriage there are many thoughts about leaving/divorcing/sharing property, then yes, it is necessary.