I keep hearing some religions push for this. I don't understand how it benefits them

you need to look at the origin. if we accept that this is written in the holy scriptures of the espective religions then the question is how did it benefit the people at the time it was written. never mind today because religions aren't free to just change scripture.

if we further assume that those scriptures were actually inspired by god or some other non human entity then the question is practically unanswerable because the benefit for that creator lie beyond our human life experience.

there are however a few potential theories for why it makes sense. for one god is described as existing beyond space and time. so god would know that birthrates were necessary to spread humanity across the planet, and maybe that was the goal. god would also know that the birthrate would eventually decline, so this would help be to counteract that decline.

we can also ask ourselves, what would be the benefit of a growing population today?

if the purpose of humanity is to push forward an ever advancing civilization, that is, to develop and to thrive, then maybe the goal is to maximize the potential of this planet to feed that many people, or to challenge us to be more efficient at food production. or thinking beyond, maybe the goal is to actually push us to colonize space.

It's significantly harder to convert an adult than it is to indoctrinate a child.