Many hold the belief that the purpose of life is to better humanity or society. Having children is a major benefit to society. Therefore, the purpose of life is to procreate because it ultimately benefits society.
Having children betters humanity. These children do not need to be geniuses who can cure cancer or solve existential dilemmas. Rather, producing offspring in any manner helps leave a positive impact on society. Procreation leaves a positive economic impact on society. A 2009 study by the Berkeley and Syracuse economics departments analyzed all possible costs and benefits of having children. The research accounted for costs to parents, greater society, and the long term for having a child. The results showed that each child holds a net benefit to society of about $217,000. Having children is an economic gain that substantially benefits society.  Procreation leaves a positive impact on society by contributing to genetic variation. As a species, we need genetic diversity to allow more room for natural selection to choose the fittest to survive. Genetic variation is the reason that we exist today. Since the purpose of life is to procreate, procreation contributes to genetic variation which ultimately betters humanity.  Even if one argues that the purpose of life is to better society, procreation at its core does benefit society. As a result, procreation is the purpose of life.
One cannot assume their posterity will better humanity. Horrible people such as serial killers, terrorists, and rapists were all children at one point, yet it is hard to argue that they have benefitted society. In addition, having children adds more people to an already overpopulated world. Overpopulation definitely does not benefit society. While procreation can have a positive impact on society, it can also have a negative impact as well. 
Rejecting the premises