The US has 25% of the world's prisoners, and over half of them are imprisoned (many for life) because of nonviolent crimes. This is a financial burden to US taxpayers who pay to keep prisons running and for the longer term imprisonments, the punishment doesn't seem to fit the crime.
Also, legalizing some of the less risky drugs that the US government bans could be a good outlet for new taxes because we know there's a demand for it.
Instead of US taxpayers losing more money to pay for prison, the people using drugs recreationally could aid in supporting the government financially by buying through non-underground sellers and the cost of running prisons would decrease because of the decrease in inmates that we would need to support. The money saved could go towards other institutions.