The prohibition on drugs puts many people in Jail - there are over 1 million prisoners for drug offences in the United States. There are even more if we consider drug-induced offences such as drug-induced theft.
These prisoners are a large strain on society which needs to support them. They also prevent more serious criminals from being jailed, which undermines the punishment and rehabilitation system of the government.
The legalisation of drugs will free the prisons from false criminals, and restore justice. And the lawful people will no longer need to support hundreds of thousands of jailed drug users and dealers.