Keeping drugs illegal harms countless users, who can't be sure the drug they're using isn't cut with something harmful or is what they though they were getting or isn't too pure (all of which are the major causes of overdoses and deaths).
Throwing people in jail also harms them and their families (and risks them getting killed or maimed during arrest or while in jail).
Jail itself is a training ground for criminals, and many inmates come out more hardened criminals, and they're forced in to committing more crimes because once they've been in jails many legal jobs are closed to them because most companies don't want to hire people with a criminal record.
Society also wastes billions of dollars on fighting the War on Drugs, which could be used much more productively, especially as the War on Drugs is so ineffective and actually hurts millions of people world-wide, causes organized crime to thrive, and even destabilizes governments when organized crime gets too powerful and starts corrupting/killing politicians, lawyers, journalists, and judges... etc.
Addiction should be treated as a medical issue, not a criminal one.