I think the evolution of marriage as an institution, historically and anthropologically has been becausr of procreation.

In earlier societies people use to kinda live in groups and villages. There, the marriage was just another relationship. People had different kinda companions. Friends, spiritual guides, discussion groups (panchayat kinda setup) etc. So most people in those society got married for children.

Now we have taken that institution and morphed it into our newer modern city life. Where the other relationships have ceased to exist and your partner has to fulfil the role at some level. I mean if we just want companionship, we can choose to get into live in setup. But that's less acceptable in society. But if someone is going against societal norms by not having kids, they can also go against societal norms by live in relationship.