The Hesperian's "Where There Is No Doctor"  offered another explaination - in some parts of the world people are too poor to not have many children. A child needs few years to grow, and then it can work for the family. This makes children an investment that returns itself with profit after few years, and it is good for the family to get many of them.
 - http://hesperian.org/books-and-resources/
The actual reasons really are 'reproduce until you get a boy' or 'have as many boys as you can' so that you can have something depend on in old age.
You would because, you are not 60 and you are not being forced to do small time jobs for paltry income to just afford food, clothing, shelter and medical care.
A person goes and works small time jobs, struggles to put food on his table. He spends almost nothing. Struggles to pay rents, or buy clothes. All this while his hand and legs have no strength. Yet his peer who has a son eats dal, roti and kabab at home watching television.
Go and all the best trying to lecture that person, why not having a son was OK.
Honestly if you were to ask me, merely the thought of being that person scares me. I would save like hell now, have enough money to feed me until I die than be in such a position when I am 60.
Honestly, the thought of becoming either(small time job vs comfortable living with son oldman) of those old persons scare me. I would rather be an oldman, more like Ron from "Into the wild"(think lifestyle business), than either of these. Unfortunately, i can't rely on Federal Health care or pension for that.