By outlawing recreational drugs, the government loses control and overview on a well-developed business. By legalizing recreational drugs, the government has a chance to regulate the quality, receive extra tax income and does not need to spend many resources on (mostly ineffective) prosecution.

If drugs were legalized would they be used more or less? Would there be more problems caused by less or would some crime levels decrease?

Even if drug laws are difficult to enforce and prosecute, they are worthwhile in terms of preserving societal norms. If recreational drugs are legalized, the stigma of using them will be somewhat reduced, and more people , who otherwise would have abstained, will engage in drug use. Since drug use affects everyone in society (in terms of lower productivity, behavioral problems, and accidents), society has a stake in reducing its use.