* Less crime because people who are addicted to drugs do not have to raise instant money for their next fix,
* Getting open, visible drug use off the streets and into a controlled supervised setting (this is not strictly part of safe supply though, but an orthogonal related service),
* Making drug use safer for the users by ensuring quality and consistent strength. Fentanyl, orphines and nitazenes are killing lots of people, and they exist on the market not because of demand from the users but because they are easier and cheaper to manufacture and smuggle than heroin.
* Making it easier to help the drug user by removing the burden and stress of obtaining the drugs. This is often an endless cycle of stress which leaves no room for getting help.
* Making it possible for drug users to be functional members of society. Not all people who are addicted to drugs need to stop, for some the safer choice is to accept that they are dependent and work around it. Taking opioids to prevent withdrawal is not incompatible with holding a job and contributing to society, committing crimes to get money for the next fix is. For many people who are addicted to opioids, abstinence programs are not the best option because the rate of falling back to using is very high, and once that happens the risk of fatal overdose is also very high due to decreased tolerance. I won't quote concrete numbers because I cannot remember them right now, but last I heard the rate of failure in abstinence programs was well above 70%.
* By supplying the drugs from sources that have no financial incentive in keeping the drug users on the drugs, you can ensure that users are exposed to all public offers to get help with their addiction or any underlying socioeconomic factors that are probably contributing to why they have become addicted in the first place.
Most criminologists will tell you that punishment is a very poor method of reducing the crime rate. I believe that to be true, and I think that a much better way is to address the underlying desperation and inequality that drives people towards crime. It doesn't mean that we should stop at the safe supply, because that would indeed be the cynical approach, we of course also have to help people overcome all the problems that are probably keeping them in their drug addiction, such as mental health issues and homelessness.