One of the benefits to legalize drugs is that the government can get more tax from them. As the government gets more tax, people can have more welfare from the country. The welfare include some public construction like high speed way, and some personal welfare like health insurance.
Another is that the black market selling will decrease, so there will be less illegal trade. Now days, people usually get drugs from black markets, and this have made illegal trades. If the government legalizes drugs, the black market selling will reduce, even disappear.
The other benefit is that there will have less prisoners. Supposing drugs has been legalized, there won't be illegal to take drugs,so there won't have the prisoners because of taking drugs. And as the crime rate cut down, our society will become more safety than ever.