Well, because many, many times people are following directions (not necessarily using spoken navigation) and like to look on the map to see when the street they're looking for is coming up. A street map without street names is a pretty ridiculous concept. I might be driving down a big, long street in my own neighborhood and looking to make a turn at a certain cross street. Since I don't know ALL of the cross streets and where they are, but I DO know that it's coming up soon, the obvious thing to do is... (go ahead, take a guess) watch the names of the streets on the map so I know when my turn is coming up