From Finn's excellent post above (which contains a lot more than just this, go read it):
But the more I allowed that openness to lead me to consider how relationships were actually being constructed, rather than just how I expected them to be constructed, I also found myself doing the opposite: considering the ways in which marriage was constructed to resemble these specific kinds of friendship.
Common friendships can be shared. In one friend one can love beauty; in another, affability; in another, generosity; in another, a fatherly affection; in another, a brotherly one; and so on. But in this friendship love takes possession of the soul and reigns there with full sovereign sway: that cannot possibly be duplicated.
Montaigne, M. de, Screech, M. A., & Montaigne, M. de. (1993). The complete Essays. Penguin Books, p. 215.
Although Montaigne distinguishes this model of loving-friendship from pederasty, he connects it to marriage
Haggerty, G. E. (2012). Gray Agonistes: Thomas Gray and Masculine Friendship. The Age of Johnson, 22, 331–XIII.
Throughout the late eighteenth century husbands were encouraged to be friends with their wives, to be as close to their wives as they had been to their intimate male friends. The effusive rhetoric of friendship was not modeled on that of romantic love, but rather romantic love took on the semiotics of friendship.
Tobin, R. (2000). Warm Brothers: Queer Theory and the Age of Goethe. University of Pennsylvania Press. https://www.jstor.org/stable/j.ctt15sk9bt, p.37.
This is not the only thing, and marriage is also an imperfect analogue for these relationships.
But! Friendship can be the model for a wide spectrum of intimacies. Some people in the past looked at friendships (platonic, romantic, heroic, what have you) and said, "Huh! We should model our most intimate and important social relationships on that!" And I think that's neat.