I recognize that we have different viewpoints on this, but in case it was a question asking about what our reasoning is, this is mine:

Primary education is an infrastructure issue. The more well educated our populace is, the better our society will be. As someone who believes that the role of government is to provide for society-wide infrastructure, educating children falls in the same vein as building roads. While there are certainly benefits to individuals provided by both of those programs, it is the betterment of the whole that is the purpose.