Easy to understand maps are a critical part of a good transit system. In fact, legibility is one of Jarrett Walker's Seven Demands of Useful Service. In Human Transit, he writes:
Legibility may seem like a minor problem compared to the others. For people who use transit only for the same trip every day, legibility is not a problem; you just learn how to make your routine trip and ignore all the other complexities. But if you want transit to be a primary transport mode for your city, then you want passengers to use transit for many purposes, not just a regular trip, and in this case the legibility of the system becomes a critical value. Anyone who has spent 15 minutes on hold waiting for a transit system’s information line (and who only called because the maps, schedules, and website were not helpful) knows that the lack of legibility really does add to travel time and crushes any sensation of freedom.
Below you can see Richmond's current system map compared to a new map that attempts to combine all of New York City's subways and buses into one—still legible—map. Both of these maps, although incredibly different in scale and scope, attempt to make it easier for riders to take full advantage of their transit system—thus providing the freedom to get wherever they need to go.
On the completely other end of the spectrum, here's a very simple yet incredibly charming map of The Pulse taken from the draft of the Richmond Transit Vision Plan (PDF). It still has the same goals: clearly convey information about how to use transit.
P.S. You should read more about the mapmaker's process of building that daunting NYC map.