Knowing that you are friends first makes it easier to rely on that person. If you cant trust them, they shouldnt be your friend in the first place. Building trust and respect are very important in a relationship, and when you already have that from the friendship, the relationship will be even stronger.

Why is friendship important in a relationship?

Being friends does more for long-term survival of a couple than anything else. It leads to better relationship outcomes over time, including a more satisfying relationship, a better friendship with ones partner, and even greater sexual satisfaction.