It's cheap housing. From a manufacturing point it's much cheaper to keep your workers in one place every day and have an assembly line making "houses", then delivering the houses rather than having the crew go to a different site each project.
"In the Great Depression in the 1930s, people started living in trailers which were designed for travelling and vacationing but out of necessity, people started to make these tiny mobile units their homes," says Andrew Hurley, author of Diners, Bowling Alleys and Trailer Parks. "They started parking them on the outskirts of cities and that's when they become associated with working class and impoverished people."