I came across an interesting article today on the website City Lab (from The Atlantic magazine) about Sanborn Maps. The article is titled Accidental Revelations of Sanborn Maps: How maps created for fire insurers show the evolution of cities. Although the article is over 6 months old, I found it very interesting.
Sanborn Maps were originally created to aid fire insurance companies. Today these historic maps can be very useful for genealogy. I encourage you to read the article and then check out Sanborn Maps at the Library of Congress website. Additionally, Lisa Louise Cook has a short video on YouTube on Using Sanborn Fire Maps for Genealogy and Family History.