What is the difference between marriage and relationship other than legalisation of relationship?

Don't get me wrong I am not against marriage, in fact I am from India, here marriage is celebrated like a festival. But I don't see the purpose of it.
What does it bring to the table which is more positive than being in a relationship?
Now I know if I ask this question here in my country my elders would smack me with all the religious books they can find and tell me relationships are not good or pure without marriage. But who decides what's pure or not?
I mean I don't believe in God but even if I did I don't think God would have time for such small things like checking a marriage certificate of a couple living together.
No offence to believers of God.

Updates
1 mo
I think I should be more specific in my question as the replies I am receiving are mostly focused on the societal perspective of marriage and I completely agree with that. Marriage is more easily acceptable by society than a relationship. But my question is what does marriage bring in extra for the couple which is not present in a relationship?
What is the difference between marriage and relationship other than legalisation of relationship?
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