The reason is that, in order to achieve the goals listed above, we want to expand transit's reach past the current social networks. Even if it's true that only spatial navigators read maps, and only "foamers" try new transit routes on a whim, these map-reading transit buffs are one way that the use of a new transit system can spread from one social network to another.
But this is only one kind of mode decision, habit. As I wrote before, there are three others: single trips, investments and subsidies. Glamour has a much bigger effect on these decisions than on habits, and so does information.
Transit information can influence how people see themselves and their neighborhoods. Think of how ubiquitous subway bullets and maps are in media relating to New York, and Underground symbols in London. The "freeways" and boulevards of Los Angeles fill a similar role. People who live in a neighborhood without a subway station, like Maspeth, think of it as a neighborhood for cars, but people who live in Ridgewood think of it as a transit neighborhood.
A person who's heard about a particular destination and wants to visit will usually take transit if it's presented as a reliable way to get there, which is why restaurant reviews and entertainment listings should include basic transit information, especially in suburban areas where transit use is not assumed.
If someone's moving to, say, Fairview, where they can't see a train line on the map, they may assume that no one takes transit and buy a car. Once they've got the car they have an incentive to use it. People who don't want to drive may avoid Fairview altogether, depriving those buses of farebox revenue.
On the other hand, if people see that there is frequent bus and van service, they may choose to move there and not own a car, contributing to the critical mass of transit riders.
Information can also influence subsidies. If politicians don't see their districts on a transit map, they may conclude that all their constituents drive. As a result, they may cut transit funding or push for driving subsidies.
Adirondacker also mentions several attempts to map buses in New Jersey that failed to stem the exodus to cars. No examples of such maps were provided, but given that people have succeeded in producing relatively readable bus maps of Queens and the Paris suburbs, I think they can do it in New Jersey.
One strategy that Jarrett has mentioned several times is a frequent network map. While I think the concept has some limitations - if your goal is for every route to have frequent service, what happens to your map when you reach that goal? - it's a good start.
Today, Jarrett mentions the excitement he felt as a teenager, watching the buses in Portland's Fareless Square:
I remember that because it's the same excitement that many of us have felt in an airport, or a great European train station, when we see a departure board showing all the exotic places to which people are departing. In the entire passenger transport experience, this is the moment that offers the most visceral sensation of freedom -- look at all the places I could go, from right here! -- and as the car industry can tell you, creating a sensation of freedom is the key to success. Urbanists need to think more about this sensation, because it could help them describe transit mobility in a way that connects it to things that we all value. Things like freedom, and joy.
I would argue that maps have the same glamour effect on people - and not just spatial navigators. Don't discount the value of that sense of the possible in transit: whether it's a map, a timetable, a departure board, a view of a bustling transfer point, or an announcer's voice on the PA, we see transit as full of possibilities. We could go to Basingstoke or Reading! Massapequa, Massapequa Park! Anaheim, Azusa and Cucamonga!