So that our opinions, actions and thoughts aren't changed/corrupted by superstition and blind faith. This is no offense to Christians or any other supporters of religion. But without religion, we wouldn't have done the Crusades, millions of people would have been spared from religiously incited genocide and war, and science and technology would be much further ahead than it is now. Religion has caused some good things, but this isn't much compared to the destruction and death that has been done in the name of god.
Education is the key. For example, in countries and states where there is no Sex Ed, people are more prone to catching STDs and having teenage pregnancies. If people aren't taught about the effects of drugs, they won't know what they're getting into.