And to prevent homeless people from attacking each other.

Parent comment was down voted but AFAIK it's correct, many homeless people avoid shelters because they consider them unsafe due to other homeless people, some of which are dangerous to others.

And that's why programs that segregate homeless people (put them on shelters, or put them on government-designated parking lots) won't ever ever succeed. You need to properly integrate the homeless into society, which in our society generally means having an apartment for each family unit.