Because if you have everybody obeying their own moral code - or immoral code - then there would be anarchy and chaos. Obviously we see this to some extent anyway, especially as the 'justice' system can often be corrupt, but having some sort of outward code to abide by means that there is some sort of protection for the innocent, better than nothing. Though I wouldn't say this necessarily makes us civilised, just restraining barbarism to a certain extent.
Education can reduce some of the evils of the world, but sadly, it's not enough. The threat of force and the use of it are necessary in order to make civilization possible. The main reason for this is that there is a gap between what we know is the right action and actually doing it. Too often we fall short of what is needed. The threat of force is not a perfect solution. But, then again, humanity isn't perfect.