I volunteered with homeless youth in LA, and it can be even more practical than that. When you're homeless, you definitionally don't have somewhere secure to sleep. When you're sleeping, you're vulnerable, to theft or violence (either by other homeless people, or the cops). So you don't sleep much, and sleep lightly. That leaves you exhausted. All the time. You need some energy to work whatever hours you can find at your woefully inadequate part time jobs, so you start using stimulants. That works for a while, but now you can't get any sleep, and the extreme stress of your perilous existence is only exasperated by stimulant abuse. At some point, depressants are the only real option to get some rest. And, as a lovely side effect, they make things feel okay. There's a reason you don't find a lot of otherwise-fine heroin addicts, unlike, say, coke or meth. Mostly, the feeling of opiates is just contentment. The addictive potential is closely tied to how miserable your unmedicated life is. Which, you know, for someone sleeping in the backseat of a 30 year old car in sketchy part of town or worse, is pretty fucking miserable.

And that's how someone can go from scared 14yo runaway to bartering favors for drugs in like, a year. Hell, I'd go even further and say that all the assholes blaming homelessness on mental health issues have the causality backwards, too. Living under constant stress would break anybody's psyche. Not to mention the fact that for most people I was helping, their lives before they were homeless weren't exactly rosy, either.