Nails were a valuable commodity during the depression and World War II, because you wasted nothing. For World War II there was another reason. Every piece of available metal was collected to melt down to make munitions for the war effort. The nails were mostly collected from construction sites, then sorted, straightened, and stored for reuse or scrapped for the metal. Child labor, in a good sense, was often used to do the sorting and straightening.
Dolores and I both had this job. Dolores in Denver and myself in Salt Lake City. We were both doing it the summer of 1941 when she was 10 and I was 9 years old.