Drugs (crack/heroin) are illegal principally to make it hard for people to get hold of them.
We don't want people to get hold of them because we don't want them to get hooked as is bad for the persons health and society at large as many people who take drugs may do crime, or hurt people around them, either as an effect taking the drugs, or as an effect of not having drugs to take.
Because drugs are illegal, people go and get their drugs from criminals. The drugs trade has lots of bad things about it and money from drugs usually goes to bad people to pay for bad things, this affects society at large.
If drugs were legalised and cheap (like alcohol), people wouldn't buy drugs from criminals or commit crime to afford drugs, this would be better for society at large. However this would have the effect that many more people will start doing drugs, this will affect their health and cause widespread addiction that would ultimately affect society at large.