Free up police time.
Get people out of prison who have committed offenses no more morally wrong than a pub landlord's selling of alcohol.
Earn enormous tax revenues from the trade.
Take the trade out of the hands of violent criminals and organised gangs.
Put an end to the occurence of petty crime to fund habits.
Make drugs safer and the dangers more understood by being open about them and regulating them.
Stop alienating and criminalising drug users and dealers.
Make the law more consistent and therefore more respected.