Decriminalizing doesn't change people with a drug problem into not having a drug problem, true. It does, at least, free them from also having a legal problem. Idea being that they can seek and get treatment for their drug problem, now. Something they can't do when it is criminal. (Indeed, reading the Wikipedia page for Portugal shows increased treatments as their first bullet in favor.)

I'd also guess that it makes it easier for treatments to be offered. As, right now, offering help there is basically aiding illegal activity.