I don't think the main beneficiary of raising children is their family. That child will one day grow into adult who will contribute to society with ideas and labour.
> Outside of nationalist 'we need to have a strong population' or basic evolutionary 'human survival' concerns I really don't see how it is a necessity for the actual people living in the country, specially all of those that don't have or want children.
Well, I don't think people who don't have or don't want children should have them. But within a society, someone needs to have them. Because someone needs to be your waiter, your janitor, your doctor, lawyer, grocer, plumber – I mean, every activity that is provided to us is provided by someone who once was a child. Everyone we work for and with was also once a child.
Given that both people who want and don't want children benefit of there being a society at all, and this society is made out of grown-up children, it's in society's interest to make it easier.
Raising a family is great but is also a burden. The benefits however are shared with the society at large. Also for people who don't want children to be able to not have them and still enjoy the benefits, it makes sense to try to spread the burden widely among society.