Safe injection sites are also a way of saying that there is someone who gives a damn about your problem, instead of seeing you as a stupid junkie. It's one of the few interactions with the government people suffering from addiction have which is devoid of judgement - that they are worth helping.
For some people, this means a lot. Some of the worst parts of addiction is how completely it destroys any sense of self-worth.
Additionally, note that pretty much all help for homeless people is heavily tied to a requirement to keep off drugs in many places, with zero tolerance policies in place. I don't know if that's the case in San Francisco, but it's a large part of the reason many homeless people are unable to access support to get them into housing.
For some people, this means a lot. Some of the worst parts of addiction is how completely it destroys any sense of self-worth.