A mimeoscope, which is basically a light table, had an electrically illuminated glass top on which the operator traced drawings onto mimeograph stencils.
The electric light was needed because the stencils were heavier and less transparent than tracing paper.
Designs, maps, and plans could be easily drawn and copied for quick production and distribution.
Customers could add these visuals to their instructions or announcements.
Those who did not have time to read the entire document would still be able to look at it and quickly know what it was about.