THEY ARE HUMAN BEINGS, I don't think that you can honestly say that you think there are useless people in the world? Are they necesarily impacting society, maybe not but that doesn't mean they don't have basic human needs and rights. If you don't want t help those outside your bubble thats col with me, but I think that calling someone less fortunate than you "useless" is one of the saddest statements I've heard!
As to how they become homeless, Many I have met said they ran into trouble with the law and when they got out they needed to get out of the situations they were in so they started traveling the US. One guy said he just got into town and just was without a home for 2 weeks until his check came in, but he worked and was hopeful that he would have a house again in 2 weeks.