I used to volunteer at a clubhouse for the mentally ill, a place where people could get food, play pool, take classes on computer skills, find employment, etc. My efforts were relatively small, but due to the fantastic management of the place, everything I did was magnified; fixing an old PC meant another 20 people might've been able to learn the skills they needed to find a job. It's been many years since then, but it's what I hold up as a good example of the sort of lever you can use in volunteering, and I just don't see an analogue for helping the homeless.
What am I missing? How can I help?