Why did the artisans use pictures on their signs?

This story is set in the time of the Revolutionary War in America.  Most of the artisans would have come to America from Europe where they learned their trades from fathers, grandfathers, or people to whom they were apprenticed in their youth.  In Europe, and also in early America, it was common for artisans to both spell out their businesses as well as have a symbol or picture which represented their business on the building.  The older buildings in places like Boston and Philadelphia still have these symbols, as do the oldest buildings in European towns and cities.  This serves the purpose of letting all potential customers know what they did at the site--both those who could read and those who could not.  Call it advertising, if you will.